nl80211: Fix external_auth check for offloaded authentication

nl80211: Fix external_auth check for offloaded authentication

Unfortunately removal of the ext_feature flag in the last revision of
the patch ended up negating the comparison and prevented the command
from being processed (either nl80211_external_auth() or
rdev_external_auth() returns -EOPNOTSUPP). Fix this by adding back the
lost '!'.

Fixes: d15ab4548f30 ("cfg80211/nl80211: Optional authentication offload to userspace")
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>