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>