iwlwifi: pcie: pull out common rfkill IRQ handling code

There's no point in duplicating exactly the same code here
for legacy and MSI-X interrupts, so pull it out into a new
function to call in both places.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>