MythMusic: fix a mix up with the genre handling in the freedb cd lookup

There's a slight difference between the output from freedb and the MusicBrainz
freedb gataway when doing a read operation which was causing the genre to get
mangled.

This also changes things so both the genre from the query and the read are
saved and we use the first non generic one we find which for MB will always be
from the query since MB doesn't support genre's so always gives us 'Unknown'
in the read :(