pwm: img: Use only a single idiom to get a runtime PM reference

Currently there are two very similar approaches in use by this driver:
img_pwm_config() uses pm_runtime_get_sync() and calls
pm_runtime_put_autosuspend() in the error path; img_pwm_enable() calls
pm_runtime_resume_and_get() which already puts the reference in its own
error path.

Align pm_runtime usage and use the same idiom in both locations.

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