V4L/DVB: cx231xx: Avoid an OOPS when card is unknown (card=0)

As reported by: Carlos Americo Domiciano <c_domiciano@yahoo.com.br>:

[  220.033500] cx231xx v4l2 driver loaded.
[  220.033571] cx231xx #0: New device Conexant Corporation Polaris AV Capturb @ 480 Mbps (1554:5010) with 6 interfaces
[  220.033577] cx231xx #0: registering interface 0
[  220.033591] cx231xx #0: registering interface 1
[  220.033654] cx231xx #0: registering interface 6
[  220.033910] cx231xx #0: Identified as Unknown CX231xx video grabber (card=0)
[  220.033946] BUG: unable to handle kernel NULL pointer dereference at (null)
[  220.033955] IP: [<ffffffffa0d3c8bd>] cx231xx_pre_card_setup+0x5d/0xb0 [cx231xx]

Thanks-to: Carlos Americo Domiciano <c_domiciano@yahoo.com.br>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>