mtd: spi-nor: guard _page_size parameter in S3AN_INFO()

The _page_size marco parameter was missing parentheses around it. Add
them.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-21-michael@walle.cc