iwlwifi: display flowhandler register when sw error or on-demand

Flowhandler handle the communication between driver and uCode, when any
uCode error happen, we also like to know what is the status of the
flowhandler; it can help to debug flowhandler related problem.

Also adding debugfs file to dump current value of flowhandler registers.

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>