Revert r21912 : "Storage API : remove undeeded target-specific functions"

After a discussion mixed on mailing list and irc, it was agreed that
more abstraction for plugins is better (so developers don't have to add
a check to HAVE_DISK_STORAGE when writing disk-specific code)

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@21923 a1c6a512-1295-4272-9138-f99709370657

apps/plugins/wavplay.c

@@ -3596,9 +3596,7 @@ int play_file(char* filename)
             if (free_space <= 0)
             {
                 filling = false;
-#ifdef HAVE_DISK_STORAGE
                 rb->storage_sleep();
-#endif
             }
             else
             {