Bluetooth: Move bt_accept_enqueue() to l2cap_sock.c

This is part of the move the parent socket usage to l2cap_sock.c

The change is safe when it comes to locking, bt_accept_enqueue() is still
protected by the parent socket lock inside the
l2cap_sock_new_connection_cb() code.

Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>