V4L/DVB (9297): cx18: Fix memory leak on card initialization failure

On error exit, the cx18_probe() function did not use the proper entry in
cx18_cards[] with kfree() when card init failed; leaking memory.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>