iwlwifi: move firmware_loading_complete to iwl_nic

Move firmware_loading_complete from iwl_priv to iwl_nic and rename it
to more accurately reflect what it does.

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>