driver: fix long ago omitted byte swap in bing-endian register write.
Even on big-endian systems ioread32/iowrite32 accesses memory-mapped register in little-endian manner to be compatible with PCI devices so the problem of missing be in iowrite32be for ctucan_hw_write32_be has not been found during MIPS Malta QEMU test, but it should be corrected for case of native CTU CAN FD core mapping on big-endian systems.