[media] cx231xx: Clear avmode bits before setting

We need to clear out the field before setting individual bits, or else we end
up with a union of whatever was there and what we are trying to set.  For
example, switching to digital mode ends up being 0x30 instead of 0x10 if we
were previously in analog tv mode.

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