Using Accelleran CU, the DRB ID is 4 (not 1), we need to set and access
the array 'drb_priority' correctly in the DU specific functions.

Before this commit, we had the following error:
[NR_MAC]   ASN1 message CellGroupConfig encoding failed (rlc-BearerToAddModList, 18446744073709551615)!