iwlwifi: Added foreward declaration for iwl_cfg in op_mode

Please merge this with "iwlwifi: op_mode holds its pointer
to the config"
Signed-off-by: NDavid Spinadel <david.spinadel@intel.com>
Signed-off-by: NWey-Yi Guy <wey-yi.w.guy@intel.com>

drivers/net/wireless/iwlwifi/iwl-op-mode.h

@@ -69,6 +69,7 @@ struct sk_buff;
 struct iwl_device_cmd;
 struct iwl_rx_cmd_buffer;
 struct iwl_fw;
+struct iwl_cfg;
 
 /**
  * DOC: Operational mode - what is it ?