We need to ensure the loads from the descriptor are done after the
MMIO store clearing the interrupts has completed, otherwise we
might still miss work.

A read back from the MMIO register will "push" the posted store and
ioread32 has a barrier on weakly aordered architectures that will
order subsequent accesses.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>