[media] v4l: subdev: Unify argument validation across IOCTLs

Separate validation of different argument types. There's no reason to do
this separately for every IOCTL.

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