gpio: mvebu: fix pwm .get_state period calculation

commit e73b0101

 upstream.

The period is the sum of on and off values. That is, calculate period as

  ($on + $off) / clkrate

instead of

  $off / clkrate - $on / clkrate

that makes no sense.

Reported-by: Russell King <linux@armlinux.org.uk>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Fixes: 757642f9

 ("gpio: mvebu: Add limited PWM support")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[baruch: backport to kernels <= v5.10]
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>