FatFs: Fix some functions to be defined only when FsWritable.
FatFs: Fix some functions to be defined only when FsWritable.

fixator/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,fixator/aprinter,fixator/aprinter,ambrop72/aprinter,ambrop72/aprinter,fixator/aprinter,ambrop72/aprinter,johnkingsley/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,fixator/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter