mtd: spi-nor: spansion: Differentiate between s25fl256s and s25fs256s

s25fs256s was identified as s25fl256s. Differentiate between them by
the Family ID using the INFO6 macro.

Fixes: b199489d ("mtd: spi-nor: add the framework for SPI NOR")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>