media: v4l2-compat-ioctl32.c: add missing documentation for a field

Fix this sparse warning:

drivers/media/v4l2-core/v4l2-compat-ioctl32.c:256: warning:
Function parameter or member 'capabilities' not described in 'v4l2_create_buffers32'

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>