mtd: spi-nor: spansion: Set ECC block size

The S28 flash family uses 2-bit ECC by default with each ECC block being
16 bytes. Under this scheme multi-pass programming to an ECC block is
not allowed. Set the writesize to make sure multi-pass programming is
not attempted on the flash.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201201102711.8727-4-p.yadav@ti.com