V4L/DVB (9802): cx18: Add module parameters for finer control over buffer allocations

cx18: Add module parameters for finer control over buffer allocations.
User now has the option of setting smaller buffers to get lower
latency transfers from the encoder.  User can also now set the number
of buffers used for a stream explicitly.

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