pwm: Restore initial state if a legacy callback fails

It is not entirely accurate to go back to the initial state after e.g.
.enable() failed, as .config() still modified the hardware, but this same
inconsistency exists for drivers that implement .apply().

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