pppoatm: allow assign only on a connected socket

The pppoatm does not check if used vcc is in connected state,
causing an Oops in pppoatm_send() when vcc->send() is called
on not fully connected socket.

Now pppoatm can be assigned only on connected sockets; otherwise
-EINVAL error is returned.

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Cc: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>