Squash casting-away-constness warnings.

BYTE2WORD() doesn't modify what its argument points to, so make that
argument a const unsigned char *.

This lets us get rid of casts that cast away constness.

Change-Id: I44a58bd3d75fc77a022b7e8f7fa9b43990bcf81c
Reviewed-on: https://code.wireshark.org/review/2876
Reviewed-by: Guy Harris <guy@alum.mit.edu>