mtd: spi-nor: Add s3an_post_sfdp_fixups()

s3an_nor_scan() was overriding the opcode selection done in
spi_nor_default_setup(). Set nor->setup() method in order to
avoid the unnecessary call to spi_nor_default_setup().

Now that the call to spi_nor_default_setup() is skipped, set
mtd.erasesize to nor->info->sector_size, as it was when
spi_nor_select_erase() was called.

No dummy byte is required for the S3AN's Random Read command (0x03),
so no need to set nor->read_dummy.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>