iwlwifi: on-screen event log dump

This feature enables the on-screen uCode event log dump. The original
method will append the event log to syslog; with this capability,
we also enable the user to write script to capture the
events which provide additional flexibility to help uCode debugging

Method
1) change to debugfs directory (sys/kernel/debug/phyX/iwlagn/data)
2) #cat log_event

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>