Bluetooth: Fix BT_ATT_ENFORCE_FLOW

This moves the processing packets of upper layers from RX thread to the
system workqueue so they have the same priority as the TX callbacks
which has the added benefit of making any protocol on top of L2CAP to
be executed using system wq stack.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>