iwlwifi: move irq to PCIe

Even if the variable might also be used by other
transports, there's no need for anything outside
of the transport itself to access it, so move it
into the private area.

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>