Low bitrate SBA tuning fix

Work on showed that there was a slight discrepancy in the worst case SCE bitrate stored in ivas_spar_br_table_consts[] in ivas_rom_com.c and the theoretical worst case SPAR MD bitrate. The theoretical worst case is 150bps higher than indicated by the worst case SCE bitrates for 13.2 and 16.4kbps SBA.