mwifiex: fix potential null dereference 'mef_entry'

drivers/net/wireless/mwifiex/cfg80211.c:2357 mwifiex_cfg80211_suspend()
  error: potential null dereference 'mef_entry'
  (kzalloc returns null)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>