On Ironlake, there is an interrupt master control bit. With the bit
disabled before clearing IIR, we do not need to handle extra interrupt
in a loop. This patch removes the loop in Ironlake interrupt handler.
It fixed irq lost issue on some Ironlake platforms.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: Zou Nan hai <Nanhai.zou@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>