Fix declaration of nandemul_WriteChunkToNAND() & nandmtd_WriteChunkToNAND().

In yaffs_guts.h the 4th argument of writeChunkToNAND is declared as "const yaffs_Spare *spare".
Do the same for nandemul_WriteChunkToNAND() & nandmtd_WriteChunkToNAND().