pwm: Introduce local struct pwm_chip in pwm_apply_state()

pwm->chip is dereferenced several times in the pwm_apply_state()
function. Introducing a local variable for it helps keeping some lines a
bit shorter.

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