iwlagn: stop interrupts when suspending

Occasionally, the device will send interrupts
while it is resuming, at a point where we are
not set up again to handle them. This causes
the core IRQ handling to completely disable
the IRQ, and then the driver won't work again
until it is reloaded/rebound.

To fix this issue disable the IRQ on suspend,
this will cause us to only get interrupts
again after we've setup everything on resume.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>