ipw2200: return error code on error in ipw_wx_get_auth()

We have assinged error code to 'ret' when get auth from some
option is not supported but never used it, but we'd better return
the error code.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: John W. Linville <linville@tuxdriver.com>