iwlwifi: split force_reset debugfs file

Split the force_reset debugfs file into two
different files:
 * "rf_reset" triggers a reset of the RF when
   written to and exposes statistics on RF
   resets when read
 * fw_restart triggers a firmware restart when
   written to and lives in the transport

This cleans up all sources of firmware restart
to originate within the transport layer and
allows us to simplify some code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>