cfg80211: avoid setting default_key if add_key fails

In cfg80211_upload_connect_keys(), we call add_key, set_default_key
and set_default_mgmt_key (if applicable) one by one. If one of these
operations fails, we should stop calling the following functions.
Because if the key is not added successfully, we should not set it as
default key anyway.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>