[media] v4l: Check pad arguments for [gs]_frame_interval

VIDIOC_SUBDEV_[GS]_FRAME_INTERVAL IOCTLs argument structs contain the pad
field but the validity check was missing. There should be no implications
security-wise from this since no driver currently uses the pad field in the
struct.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>