corrected crc checksum calculation

...o/libraries/rdma_icrc_external/src/vhdl/crc_generator.vhd

...mo/libraries/rdma_packetiser/cocotb/tb_rdma_packetiser.py

...mo/libraries/rdma_packetiser/src/vhdl/rdma_packetiser.vhd

...a_packetiser/src/vhdl/rdma_packetiser_assemble_header.vhd

...ibraries/rdma_packetiser/src/vhdl/rdma_packetiser_pkg.vhd

...packetiser/tb/vhdl/tb_rdma_packetiser_assemble_header.vhd