mtd: spi-nor: cast to u64 to avoid uint overflows

The "params->size" is defined as "u64".
And "info->sector_size" and "info->n_sectors" are defined as
unsigned int and u16.
Thus, u64 data might have strange data(loss data) if the result
overflows an unsigned int.
This patch casts "info->sector_size" to an u64.

Signed-off-by: huijin.park <huijin.park@samsung.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>