[media] v4l: ioctl: Clear the v4l2_pix_format_mplane reserved field

The S_FMT and TRY_FMT handlers in multiplane mode attempt at clearing
the reserved fields of the v4l2_format structure after the pix_mp
member. However, the reserved fields are inside pix_mp, not after it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran@bingham.xyz>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>