V4L/DVB (7891): cx18/ivtv: fix open() kernel oops

Upon error conditions in cx18/ivtv_probe(), the code at the 'err:' label
leaves a NULL entry in cx18/ivtv_cards[]. This can cause a NULL pointer
de-reference in cx18/ivtv_v4l2_open() which is fixed by this patch.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>