V4L/DVB (9891): cx18 Replace magic number 63 with CX18_MAX_FW_MDLS_PER_STREAM

Removed magic number that referred to firmware limit on the number of
MDLs the firmware can maintain for any stream at any one time.

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