mtd: spi-nor: Fix S3AN addressing calculation

The page calculation under spi_nor_s3an_addr_convert() was wrong. On
Default Address Mode we need to perform a divide by page_size.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>