V4L/DVB (8771): cx18: Remove redundant struct cx18_queue length member.

cx18: Remove redundant struct cx18_queue length member. It can be trivially
computed from queue->buffers * stream->buf_size, if ever really needed.

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