IPX: Use more the nd_uint32_t type

IPX: Use more the nd_uint32_t type

In 'struct ipxHdr'.
This avoid some 'undefined-behavior' warnings with clang and option
-fsanitize=undefined enabled.

print-ipx.c:79:2: runtime error: member access within misaligned address
0x61d00001ea91 for type 'const struct ipxHdr', which requires 2 byte alignment
0x61d00001ea91: note: pointer points here
 54 e0 e0  03 ff ff 00 50 00 14 00  00 00 00 ff ff ff ff ff  ff 04 55 a8 f8 79 67 00  00 00 00 00 01
              ^
SUMMARY: AddressSanitizer: undefined-behavior print-ipx.c:79:2 in