staging: rtl8188eu: don't define OnAuth() in non-AP mode

If CONFIG_88EU_AP_MODE is undefined, OnAuth() is never referenced.

Fixes warning:
  drivers/staging/rtl8188eu/core/rtw_mlme_ext.c:2725:21: warning: ‘OnAuth’ defined but not used [-Wunused-function]

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Fixes: 68345dd7 ("staging: rtl8188eu: rtw_mlme_ext.c: unexport message callbacks")
Cc: Greg Kroah-Hartman <gregkh@linux.com>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>