mtd: spi-nor: xilinx: Use manufacturer late_init() to set setup method

post_sfdp was misleading in this case, as SFDP is not supported by
xilinx. Plus, there's no fixup here, just setting the correct
setup method, as required by xilinx parts.

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/20211029172633.886453-10-tudor.ambarus@microchip.com