pwm: Fix order of freeing resources in pwmchip_remove()

pwmchip_add() calls of_pwmchip_add() only after adding the chip to
pwm_chips and releasing pwm_lock. So the proper order in
pwmchip_remove() is to call of_pwmchip_remove() before taking the mutex
and removing the chip from pwm_chips. This way pwmchip_remove() releases
the resources in reverse order compared to pwmchip_add() requesting
them.

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