[media] adv7180: fix querystd() method for no input signal

When the input signal is not detected querystd() method should return
V4L2_STD_UNKNOWN instead of previously latched analog video standard.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>