media: tvp5150: add querystd

Add the querystd video_op and make it return V4L2_STD_UNKNOWN while the
TVP5150 is not locked to a signal.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>