V4L/DVB (9515): cx18: Use correct Mailbox IRQ Ack values and misc IRQ handling cleanup

cx18: Use correct Mailbox IRQ Ack values and misc IRQ handling cleanup.
The SCB field definitions for Ack IRQ's for mailboxes were inconsistent with
the bitmasks being loaded into those SCB fields and the SW2 Ack IRQ handling
logic.  Renamed fields in SCB to make things consistent and did misc IRQ
handling cleanups: removing legacy ivtv dma_reg_lock, HPU IRQ flags, etc.

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