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/video.c

@@ -681,9 +681,7 @@ int PlayTick(int fd)
 #endif
             )
         {
-#ifdef HAVE_DISK_STORAGE
             rb->storage_sleep(); /* no point in leaving the disk run til timeout */
-#endif
             gPlay.bDiskSleep = true;
         }