[media] v4l2-dv-timings: skip standards check for V4L2_DV_BT_CAP_CUSTOM

Skip validating the standards field in v4l2_valid_dv_timings() if the
V4L2_DV_BT_CAP_CUSTOM capability is set, since that implies that
non-standard timings are allowed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>