bugfix: add handling of DFT 768 everywhere it's needed

In the eNB it's not really used for the moment because there
is a limitation to uplink scheduling. In
openair2/LAYER2/MAC/eNB_scheduler_ulsch.c we have:

            while (((rb_table[rb_table_index] > (N_RB_UL - 1 - first_rb[CC_id]))
                    || (rb_table[rb_table_index] > 45))

That limits uplink scheduling to at most 44 RBs. The 768 case is for
64 RBs.