V4L/DVB (9894): cx18: Use a known open task handle when setting stream CX2341x parameters

cx18: Use a known open task handle when setting stream CX2341x parameters
Sometimes, we might only have VBI or YUV stream open instead of an MPEG stream.
Let's make sure we use a valid task handle to perform the CX2341x control
settings.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>