V4L/DVB (9725): cx18: Remove unnecessary MMIO accesses in time critical irq handling path

Remove unnecessary MMIO accesses in time critical irq handling path.  Also
ensured that the mailbox ack field is read in last, so we know for sure if we
have a stale mailbox or not on receipt.

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