[media] adv7183: fix querystd

If no signal is detected, return V4L2_STD_UNKNOWN. Otherwise AND the standard
with the detected standards.
Note that the v4l2 core initializes the std with tvnorms before calling the
querystd ioctl.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Scott Jiang <scott.jiang.linux@gmail.com>
Acked-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>