bugfix: fix allocation of RBGs for retransmission

When there is a retransmission, we want to use the same number of RBs.

The RBs are allocated by groups (RBGs).

In some cases (like a cell with 25 PRBs), the last RBG contains less RBs. So in case of a retransmission, if this last RBG was used before we need to reuse it to have the same number of RBs. If not, we will use more RBs for the retransmission than the previous transmission.

In an experiment with two UEs (quectel modules) it was seen that when a retransmission happens with a different number of RBs then the UE is not happy at all, leading to way more NACKs in the harq processes than should happen (actually the first NACK was not a NACK at all, but the eNB did interpret it as a NACK; so the next transmission should simply be discarded by the UE that successfully received the first transmission; instead the UE fails to decode the data and sends a NACK, a real one this time).

Maybe it's not the correct solution, but it improve things, there are much less NACKs.