The constant RLC_UM_SEGMENT_NB_MAX_LI_PER_PDU is set to 24. This is
too low. Running a downlink TCP iperf with a 10MHz or 20MHz cell
leads to lots of the following warning in the log:

[RLC][W][SN 792] Bad RLC header! Discard this RLC PDU (size=618)

The problem is that the uplink RLC PDU contains a lot of TCP ack.
We can see much more than 24 of them. The RLC layer is not happy
and discards the PDU.

Putting 256 for RLC_UM_SEGMENT_NB_MAX_LI_PER_PDU (arbitrarily chosen,
big but not too big) solves the problem, at least for 10MHz.