mwifiex: fix possible memory leak in mwifiex_cfg80211_start_ap()

memory is malloced in mwifiex_cfg80211_start_ap() and should be
freed before leaving from the error handling cases, otherwise it
will cause memory leak.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>