bluetooth: disable BT_ATT_ENFORCE_FLOW for BlueNRG devices

The firmware on these devices seems to have a bug that can cause reordering
of received packets. This can lead to new GATT requests being received
before the acknowledgement of the previous GATT response, accompanied by
log messages like the following:

<wrn> bt_att: bt_att_recv: Ignoring unexpected request

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>