iwlwifi: add function to reset/tune radio if needed

Adding "radio reset" function to help reset and stabilize the radio.

During normal operation, sometime for unknown reason, radio encounter
problem and can not recover by itself; the best way to
recover from it is to reset and re-tune the radio. Currently, there is
no RF reset command available, but since radio will get reset when
switching channel, use internal hw scan request to force radio
reset and get back to normal operation state.

The internal hw scan will only perform passive scan on the first
available channel (not the channel being used) in associated state. The
request should be ignored if already performing scan operation or STA is
not in associated state.

Also include an "internal_scan" debugfs file to help trigger the
internal scan from user mode.

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>