pwm: tiecap: Drop .free() callback

ecap_pwm_free is only called when a consumer releases the PWM (using
pwm_put() or pwm_free()). The consumer is expected to disable the PWM
before doing that. It's not clear if a warning about that is justified, but
if it is this is independent of the actual driver and can better be done in
the core. Also if there is a good reason it's wrong to disable the hardware
and so the call to pm_runtime_put_sync() should be dropped. Moreover there
is no matching pwm_runtime_get call and so the runtime usage counter might
become negative.

Fixes: 8e0cb05b ("pwm: pwm-tiecap: PWM driver support for ECAP APWM")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>