mtd: spi-nor: sst: Use manufacturer late_init() to set _write()

Setting the correct nor->mtd._write in a fixup hook was misleading,
since this is not a fixup, just a specific setting for SST, that differs
from the SPI NOR core default init.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20211029172633.886453-11-tudor.ambarus@microchip.com