hwmon: pwm-fan: Use platform_get_irq_optional()

The PWM fan interrupt is optional, so we don't want an error message in
the kernel log if it wasn't specified.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://lore.kernel.org/r/20190828083411.2496-2-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>