lm8323 pwm fixes

This patch fixes two problems in LM8323 PWM control. One is that
locking is needed when setting and reading pwm->desired_brightness
asynchronously. The other is that LM8323 may stop a PWM script only
after the current instruction has finished. If it is a long RAMP,
the chip would keep executing the old instruction and new
settings were effectively ignored.

Signed-off-by: Timo Karjalainen <timo.o.karjalainen@nokia.com>
Signed-off-by: Daniel Stone <daniel.stone@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>