vcl: set min threshold for tx ntf

vcl: set min threshold for tx ntf

Make sure there's at least 3% space in the tx fifo before notifying vcl
of a tx event. The threshold is somewhat arbibrary but for a 4M fifo, it
now means that ~120kB of space should be available.

Should help minimize the amount of tx notifications generated by
session layer when apps are faster.

Type: improvement

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I11dd0931dca8f989000a2481f1f495bd267589c4