[media] ml86v7667: fix the querystd implementation

The *std should be set to V4L2_STD_UNKNOWN, not the function's return code.
Also, *std should be ANDed with 525_60 or 625_50.

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