PWM core was converted to u64 by the commit a9d887dc ("pwm: Convert
period and duty cycle to u64") but did not change the duty_cycle_store()
so it will error out if trying to pass a numeric string bigger than
2^32-1.

Fix this by using u64 and kstrtou64() in duty_cycle_store().
Signed-off-by: NJarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: NGuru Das Srinagesh <gurus@codeaurora.org>
Acked-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>