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>