Squelch casting-away-constness warnings.

The pStr argument to dissect_mq_charv() isn't modified (and always
points to a character string), so make it a "const char *", and
eliminate the casts to "guint8 *" in calls to it.

Change-Id: I21dad38c41324528be297a8ddc1854beff2276db
Reviewed-on: https://code.wireshark.org/review/2877
Reviewed-by: Guy Harris <guy@alum.mit.edu>