[media] v4l2-dv-timings: only check standards if non-zero

If one or both of the timings being compared have the standards field
with value 0, then accept that. Only check for matching standards if
both timings have actually filled in that field.

Otherwise no match will ever be found since when timings are detected
the standards field will typically be set to 0 by the driver.

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