leds: st1202: Program the channel current for hardware patterns

A channel's output is ILED x Pattern_PWM / 4095, so a pattern is only
visible while the analog current register holds a non-zero value.
st1202_led_pattern_set() programs the PWM slots and starts the sequencer
but never writes ILED, so a channel whose current is zero stays dark
while pattern_set() reports success.

Zero is reached by ordinary use. Writing 0 to brightness sets it, and so
does switching away from the pattern trigger, because
pattern_trig_deactivate() ends with led_set_brightness(LED_OFF). A
sequence as simple as

  echo none > trigger
  echo pattern > trigger
  echo "255 500 0 500" > hw_pattern

therefore programs a pattern that never lights up.

Program the current in pattern_set() and release it in pattern_clear(),
so the pattern path owns ILED for as long as a pattern is loaded.
Releasing it matters because the trigger calls pattern_clear() before it
parses a new pattern: without that, a rejected or empty hw_pattern write
would leave the channel at full current with every PWM slot at full
scale, latching the LED on. Clearing before the slots are raised also
avoids a brief full-brightness flash on every reprogram.

A current of zero is not enough to release the channel, though: the
datasheet notes that it does not switch the channel off but leaves a
small uncontrolled current, which shows as a faint glow. pattern_clear()
therefore also disables the channel, and pattern_set() enables it again.
Both are done before the sequencer is stopped, so an I2C error partway
through a clear cannot leave the channel lit.

Probe clears every channel, so the LEDs now reach user space switched
off. Until now they came up lit at the chip's power-on current, because
pattern_clear() fills every slot at full scale. Probe no longer enables
the channels itself either: st1202_setup() has already disabled them
all, and each path that lights a channel enables it when needed.

The current is programmed to the same max_brightness the PWM values are
scaled against, so a pattern step of N produces the same output as
writing N to brightness, and a board declaring a lower ceiling through
the max-brightness property keeps it on both paths.

Note that this makes a hardware pattern take the channel to its maximum
current, so brightness reads back as max_brightness once a pattern is
loaded and as zero once it is cleared. The pattern's own values, not the
last brightness written, decide what the channel shows while a pattern
is running.

Fixes: 259230378c65 ("leds: Add LED1202 I2C driver")
Signed-off-by: Manuel Fombuena <fombuena@outlook.com>
Assisted-by: LLM
Link: https://patch.msgid.link/GV1PR08MB84978D7C1CC0367D1818093DC5BA2@GV1PR08MB8497.eurprd08.prod.outlook.com
Signed-off-by: Lee Jones <lee@kernel.org>

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