Bluetooth: ATT: Enforce flow for incoming requests & indications

It's not valid for a peer to send another request before getting the
response to a previous one, or to send another indication without
getting the confirmation to a previous one. Take advantage of the
recently introduced TX callback to track when it's ok to accept these
ATT PDUs again.

The HCI USB transport has a potential issue here since a race
condition can occur between the ACL data and HCI event endpoints,
leading to dropping data when in fact both peers were behaving
correctly. To avoid hitting this issue, disable the flow enforcement
by default on the qemu targets that commonly use a USB-based
controller on the host OS.

Change-Id: I2791aaec6f6c0f8fd78a9a809a25e3ce129106c7
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>