iwlwifi: use iwlagn_fw_error instead of iwl_nic_error

In the process, make iwlagn_fw_error
a non-static function, as it is used
by more than one file.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>