There was a problem with SN size. It was hardcoded to 12 bits (so max
sn = 4095) but we configure the UE with 18 bits. After a while, iperf
didn't work anymore because of this (what a surprise!).

So let's get the correct value from pdcp_Config.

And while we're at it, let's store also discard_timer and t_reordering
that will be used later by a proper NR PDCP layer (which is currently
work in progress).