media: tvp5150: fix standard autodetection

Make sure to not overwrite decoder->norm when setting the standard
in hardware, but only when instructed by V4L2 API calls.

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>