V4L/DVB (11615): cx18: Rename the work queue to "in_work_queue"

Rename the work queue to "in_work_queue" to indicate it is handling
incoming mailbox commands.  This is preparation for adding a work queue
for handling deferrable outgoing mailbox commands.

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