cfg80211: don't set privacy w/o key

When wpa_supplicant is used to connect to open networks,
it causes the wdev->wext.keys to point to key memory, but
that key memory is all empty. Only use privacy when there
is a default key to be used.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Tested-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>