Reduce function calls in data framer

Same function is called multiple times in the pMain to define the array index for constants. See here: https://gitlab.cern.ch/cms-tracker-phase2-data-processing/BE_firmware/dtc/-/blob/master/dtc-fe/firmware/hdl/lpgbt-framer/emp_rx_user_data_framer.vhd?ref_type=heads#L99