cfg80211: check wdev->netdev in connection work

If a P2P-Device is present and another virtual interface triggers
the connection work, the system crash because it tries to check
if the P2P-Device's netdev (which doesn't exist) is up. Skip any
wdevs that have no netdev to fix this.

Cc: stable@vger.kernel.org
Reported-by: YanBo <dreamfly281@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>