mac80211_hwsim: fix error return code in init_mac80211_hwsim()

Fix to return -ENOMEM in the netdev alloc error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>