remove numerical genre and use genre_string consistently:
- fix spurious display of "blues" genre for missing genre tag 
- simplify code/use less code
- numerical->string conversion only once instead of at every use

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@12552 a1c6a512-1295-4272-9138-f99709370657
6 files changed