iwlwifi: adding interrupt counter in debugfs for debugging

This patch adds interrupt statistics report to debugfs, this can help to
understand number of interrupts happened which including HW/SW error for
easier and better debugging.

in /sys/kernel/debug/ieee80211/phyN/iwlagn/data directory
use "cat interrupt" to view the current interrupt counter
use "echo 0 > interrupt" to clear interrupt counter

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>