leds: st1202: Do not set brightness from atomic context

st1202_brightness_set() takes the chip mutex and performs I2C transfers,
both of which can sleep, but it is installed as the brightness_set
callback. led_set_brightness_nopm() calls that op directly on the
assumption that it never sleeps, including from softirq context, so the
driver can sleep in atomic context.

The driver also installs brightness_set_blocking, but the core only
falls back to it when brightness_set is absent, so st1202_led_set() was
never reached. It merely turned the channel on or off and ignored the
requested brightness, so dropping brightness_set on its own would have
lost brightness control entirely.

Move the register programming into st1202_led_set() so the blocking
callback honours the brightness value, and drop brightness_set. The core
now defers the update to a work queue, where sleeping is allowed. Errors
from the register writes are propagated rather than discarded.
st1202_led_set() was the last user of the locking st1202_channel_set()
wrapper, which is removed.

Programming every PWM slot to full scale is kept: it is what makes the
brightness visible while the global sequencer is running, without having
to stop the sequencer and disturb the other channels.

Fixes: 259230378c65 ("leds: Add LED1202 I2C driver")
Signed-off-by: Manuel Fombuena <fombuena@outlook.com>
Assisted-by: LLM
Link: https://patch.msgid.link/GV1PR08MB8497CF8FB57A272EB671F0C6C5BA2@GV1PR08MB8497.eurprd08.prod.outlook.com
Signed-off-by: Lee Jones <lee@kernel.org>

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