media: tvp7002: fix uninitialized variable warning

tvp7002.c: In function 'tvp7002_g_register':
tvp7002.c:691:11: warning: 'val' may be used uninitialized in this function [-Wmaybe-uninitialized]
  691 |  reg->val = val;
      |  ~~~~~~~~~^~~~~

Just return without setting 'reg' if tvp7002_read returns an error.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>