backlight: mp3309c: Use pwm_apply_might_sleep()

pwm_apply_state() is deprecated since commit c748a6d7 ("pwm: Rename
pwm_apply_state() to pwm_apply_might_sleep()"). This is the final user
in the tree.

Signed-off-by: Sean Young <sean@mess.org>
Tested-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240128154905.407302-1-sean@mess.org


Signed-off-by: Lee Jones <lee@kernel.org>