virt_wifi: fix error return code in virt_wifi_newlink()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: c7cdba31

 ("mac80211-next: rtnetlink wifi simulation device")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>