the Tx SDU has been set to 128 PDUs because this gives approx.
150kByte UL buffer state (max value for Rel 8 BSR).
Previously, however, we've always reported too much pending UL
data because we weren't subtracting the transmitted data from
the reported data.
with this newer BSR reporting, however, even with full buffers we would
never report full buffers because the queues size is too short.
This seems to cause issues for iperf TCP sessions not resulting
in full DL rate, see isse #1703. It seems that the TCP bandwitdh
probing works better with larger buffers (in our case this can
only be achieved with more PDUs in the pool).
This should fix#1703