pwm: Narrow scope of struct pwm_device pointer

In the expression determining the size of the allocation for chip->pwms
it's more natural to use sizeof(*chip->pwms) than sizeof(*pwm). With
that changed, the variable pwm is only used in a for loop and its scope
can be reduced accordingly.

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