media: tvp5150: initialize subdev before parsing device tree

There are several debug prints in the tvp5150_parse_dt() function, which
do not print the prefix, because the v4l2_subdev is not initialized, yet.

Initialize the v4l2_subdev before parsing the device tree to fix the
debug messages.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>