mtd: cfi_flash: Write buffer size adjustment for M29EW Numonyx devices

This patch addjusted the write buffer size for M29EW devices those
are operated in 8-bit mode.

The M29EW devices seem to report the CFI information wrong when
it's in 8 bit mode.

There's an app note from Numonyx on this issue and there's a patch
in the open source as well for Linux, but it doesn't seem to be in mainline.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Tested-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>