leds: st1202: Scale pattern brightness to the 12-bit PWM range

The LED1202 PWM registers are 12 bits wide, but st1202_led_pattern_set()
writes the 8-bit brightness value carried by struct led_pattern into
them unscaled. A pattern step asking for full brightness therefore
drives the channel at 255 out of 4095, about 6% duty cycle, so a pattern
is dimmer than the same brightness set directly.

Scale the value to the full PWM range before programming it.

Scale against max_brightness rather than the register maximum. The LED
core takes max_brightness from the DT max-brightness property, so a
board can declare a ceiling below full scale, and the pattern trigger
already rejects steps above it. Scaling against a hardcoded 255 would
leave such a board unable to reach its own declared maximum from the
pattern path while the brightness path reaches it.

Scaling divides by max_brightness. The LED core replaces a zero
max-brightness from the device tree with LED_FULL, so the divisor should
never be zero, but the helper returns zero rather than rely on that.
max_brightness is capped at U8_MAX, the driver's own maximum, so a
device tree declaring more than the part supports is treated as full
scale. struct led_pattern brightness is signed, so clamp both ends and
floor a negative value to LED_OFF rather than let it wrap.

Document that a step is relative to max_brightness, since the range
given in the documentation no longer holds when the device tree sets
max-brightness.

Fixes: 259230378c65 ("leds: Add LED1202 I2C driver")
Signed-off-by: Manuel Fombuena <fombuena@outlook.com>
Assisted-by: LLM
Link: https://patch.msgid.link/GV1PR08MB8497B938D3592582E17CB757C5BA2@GV1PR08MB8497.eurprd08.prod.outlook.com
Signed-off-by: Lee Jones <lee@kernel.org>

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