V4L/DVB (10218): cx23885: Fix Oops for mixed install of analog and digital only cards

Analog support for HVR-1250 has not been completed, but does exist for
the HVR-1800.

Since both cards use the same driver, it tries to create the analog
dev for both devices, which is not possible.

This causes a NULL error to show up in video_open and mpeg_open.

-Mark

Iterations through the cx23885_devlist must check for NULL
pointers as some supported devices only have DVB support at the moment.
Mark Jenks encoutered an Oops in a system with both an HVR-1250 and HVR-1800
installed.

-Andy

Reported-by: Mark Jenks <mjenks1968@gmail.com>
Tested-by: Mark Jenks <mjenks1968@gmail.com>
Signed-off-by: Mark Jenks <mjenks1968@gmail.com>
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>