V4L/DVB (8701): cx18: Add missing lock for when the irq handler manipulates the queues

cx18: Add missing lock for when the irq handler manipulates the queues.  This
was a potential source of stream queue corruption.  Also changed the name of
cx18_queue_find_buf() to cx18_queue_get_buf_irq().

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