leds: lm3601x: Calculate max_brightness and brightness properly

leds: lm3601x: Calculate max_brightness and brightness properly

The torch_current_max should be checked not exceeding the upper bound.
If it does, throw a warning message and set to LM3601X_MAX_TORCH_I_UA.

LM3601x torch brigthness register (LM3601X_LED_TORCH_REG) takes 0 as the
minimum output (2.4 mA). However, 0 of led_brightness means LED_OFF.
Adding a -1 offset to brightness before writing to brightness
register, so when users write minimum brightness (1), it sets lm3601x
output the minimum.

Signed-off-by: Jack Chen <zenghuchen@google.com>
Link: https://lore.kernel.org/r/20240704191500.766846-1-zenghuchen@google.com
Signed-off-by: Lee Jones <lee@kernel.org>