V4L/DVB (13433): cx18: Remove duplicate list traversal when processing incoming MDLs

Update the incoming MDL's buffers' bytesused and sync the buffers for the cpu
in one pass instead of two.

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