mtd: m25p80: set writebufsize

Using UBI on m25p80 can give messages like:

    UBI error: io_init: bad write buffer size 0 for 1 min. I/O unit

We need to initialize writebufsize; I think "page_size" is the correct
"bufsize", although I'm not sure. Comments?
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@kernel.org [2.6.38+]
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/devices/m25p80.c

@@ -932,6 +932,7 @@ static int __devinit m25p_probe(struct spi_device *spi)
 	ppdata.of_node = spi->dev.of_node;
 	flash->mtd.dev.parent = &spi->dev;
 	flash->page_size = info->page_size;
+	flash->mtd.writebufsize = flash->page_size;
 
 	if (info->addr_width)
 		flash->addr_width = info->addr_width;