nl80211: Use netlink connect socket for disconnect (ext auth)

nl80211: Use netlink connect socket for disconnect (ext auth)

When external authentication is used, a specific netlink socket is used
to send the connect command. If the same socket is not used for
disconnect command, cfg80211 will discard the command. This constraint
was added into the kernel in commit bad292973363 ("nl80211: Reject
disconnect commands except from conn_owner"). That requires an update
for the hostap.git commit 40a68f33844f ("nl80211: Create a netlink
socket handle for the Connect interface").

Add a new flag into struct i802_bss to indicate if the special
nl_connect socket was used for the connect command. When sending
disconnect command this flag is tested to select the correct socket.

Signed-off-by: Cedric Izoard <cedric.izoard@ceva-dsp.com>