LE: Release transmit buffer credits

When an ACL link goes down, some controllers do not send a "number of
completed packets" update for the last ACL packets sent from the host to
the controller. This could result in incorrect L2CAP transmit window
calculations and no transmit credit to be available to send data to the
controller.
This patch restores transmit buffer creedits regardless of the
controller behaviour.
bug 9474159

Change-Id: I2540131034271a0269fa50698f370bceb7747e2d