V4L/DVB (6188): Avoid a NULL pointer dereference during mpeg_open()

Bug: With a hardware encoder board installed as cx88[1] and a
non-encoder boards installed as cx88[0], an OOPS is generated
during cx8802_get_device() called from mpeg_open().

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>