[media] cx231xx: fix race condition in DVB initialization

Fix case where analog calls come in while the DVB side of the board is still
initializing.  This patch is actually just an exact port of the same patch
made by Mauro to em28xx in hg rev 14762.

Signed-off-by: Devin Heitmueller <dheitmueller@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>