Bluetooth: Drop packets on ATT fixed channel on BR/EDR

The ATT fixed channel is only valid when using LE connections. On
BR/EDR it is required to go through L2CAP connection oriented
channel for ATT.

Drop ATT packets when they are received on a BR/EDR connection.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>