mtd: OneNAND: initialize mtd->writebufsize to let UBI work

io_init checks this value and fails with "bad write buffer size 0 for
2048 min. I/O unit"
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>

drivers/mtd/onenand/onenand_base.c

@@ -2657,6 +2657,7 @@ int onenand_probe(struct mtd_info *mtd)
 	mtd->_sync = onenand_sync;
 	mtd->_block_isbad = onenand_block_isbad;
 	mtd->_block_markbad = onenand_block_markbad;
+	mtd->writebufsize = mtd->writesize;
 
 	return 0;
 }