Add back a value_string conversion lost in g58cc452.

I don't have a capture to check but this appears to be correct.

Change-Id: I93405d1e3ec4aac3761d06e257db3ec2e32b2fc2
Reviewed-on: https://code.wireshark.org/review/4587
Reviewed-by: Gerald Combs <gerald@wireshark.org>