V4L/DVB (11619): cx18: Simplify the work handler for outgoing mailbox commands

Simplify the way outgoing work handler gets scheduled to send empty buffers
back to the firmware for use.  Also reduced the memory required for scheduling
this outgoing work, by using a single, per stream work object.

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