Don't starve a BT ACL link with 0 xmit quota

With new link management of controller buffers between
different ACL connections, it is possible that a ACL
conn would get 0 buffers - thus no profile can run.
This affects classic and LE ACL connections.

This work around allows a floor of 1 buffer as the
floor for all connection, irrespective of controller
capability.

Bug: 17976311
Change-Id: I7cd0d34af47c441c5ea7f2b836b591a4fec62ad1