V4L/DVB (9722): cx18: Convert per stream queue spinlocks into mutexes

Convert the per stream queue spinlocks into mutexes.  All queue
manipulation happens via the work queue or system calls into the driver, and
not in an interrupt context.  This reduces the amout of time the cx18 driver
keeps interrupts disabled.

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