pwm: Zero-initialize the pwm_state passed to driver's .get_state()

This is just to ensure that .usage_power is properly initialized and
doesn't contain random stack data. The other members of struct pwm_state
should get a value assigned in a successful call to .get_state(). So in
the absence of bugs in driver implementations, this is only a safe-guard
and no fix.

Reported-by: Munehisa Kamata <kamatam@amazon.com>
Link: https://lore.kernel.org/r/20230310214004.2619480-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>