Bluetooth: Don't lock sock inside l2cap_get_sock_by_scid()

Fix an locking issue with the new l2cap_att_channel(). l2cap_att_channel()
was trying to lock a locked socket.

Reported-by: Anderson Lizardo <anderson.lizardo@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>