backlight: Pass device through notify callback in the pwm driver

Add the device to the notify callback's arguments in the PWM backlight
driver. This brings the notify callback into line with the other
callbacks defined by this driver.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>