video: backlight: fix pwm inversion

set_pwm() will always fail with -ENOSYS if pwm_ops set_invert() is
not implemented, leaving the backlight dark. Fix this by returning
no error if set_invert() is not implemented and no polarity change
is requested.

Fixes: 57e77754 ("video: backlight: Parse PWM polarity cell")
Signed-off-by: Marc Dietrich <marvin24@gmx.de>