V4L/DVB (12112): cx231xx: fix uninitialized variable.

The variable 'rc' could be used uninitialized in the cx231xx_capture_start
function. Sri informed me that it should be initialized to -1.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>