Simplify ue get sdu pusch

in UE, nr_ue_get_sdu() gets PUSCH data from RLC and adds BSR (the ask for PUSCH grants in future slots), then finish by padding
with oai/develop, the packets are "malformed" in wireshark

the code is very hard to understand
this MR simplifies the code, and make it correct in wireshark

i can't say where are the bugs in present develop code, it is too badly designed