media: gscpa/stv06xx: fix memory leak

For two of the supported sensors the stv06xx driver allocates memory which
is stored in sd->sensor_priv. This memory is freed on a disconnect, but if
the probe() fails, then it isn't freed and so this leaks memory.

Add a new probe_error() op that drivers can use to free any allocated
memory in case there was a probe failure.

Thanks to Pavel Skripkin <paskripkin@gmail.com> for discovering the cause
of the memory leak.

Reported-and-tested-by:  <syzbot+e7f4c64a4248a0340c37@syzkaller.appspotmail.com>

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