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>
diff --git a/drivers/mtd/onenand/onenand_base.c b/drivers/mtd/onenand/onenand_base.c
index d194d97..0e35dc5 100644
--- a/drivers/mtd/onenand/onenand_base.c
+++ b/drivers/mtd/onenand/onenand_base.c
@@ -2657,6 +2657,7 @@
 	mtd->_sync = onenand_sync;
 	mtd->_block_isbad = onenand_block_isbad;
 	mtd->_block_markbad = onenand_block_markbad;
+	mtd->writebufsize = mtd->writesize;
 
 	return 0;
 }