nr pdcp: use pdcp config

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).