pwm: Fix setting period with #pwm-cells = <1> and of_pwm_single_xlate()

For drivers making use of of_pwm_single_xlate() (i.e. those that don't
pass a hwpwm index) and also don't pass flags, setting period was
wrongly skipped. This affects the pwm-pxa and ti-sn65dsi86 drivers.

Reported-by: Karel Balej <balejk@matfyz.cz>
Link: https://lore.kernel.org/r/D05IVTPYH35N.2CLDG6LSILRSN@matfyz.cz
Fixes: 40ade0c2

 ("pwm: Let the of_xlate callbacks accept references without period")
Tested-by: Karel Balej <balejk@matfyz.cz>
Link: https://lore.kernel.org/r/20240329103544.545290-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>