iwlwifi: make iwl_nic_error static

iwl_nic_error is used in iwl-agn.c only, move
it there and make it static.

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>