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: 40cbfa90218b ("cfg80211/nl80211: Optional authentication offload to userspace")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: db8d93a7a355121d49777c059afbca23c53c8628
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-Fixed: 2210017
Change-Id: I41f7c25d68a5060d7b5e24fb5bd164e063311ed0
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>