pwm: Clarify which state pwm_get_state() returns

Given that lowlevel drivers usually cannot implement exactly what a
consumer requests with pwm_apply_state() there is some rounding
involved.

pwm_get_state() returns the setting that was requested most recently by
the consumer (opposed to what was actually implemented in hardware in
reply to the last request). Clarify this in the function kerneldoc.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>