mtd: spi-nor: Fix comment of spi_nor_find_best_erase_type()

Erase types are sorted *smallest* type first, refer to
spi_nor_sort_erase_mask().

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>