mtd: spi-nor: fix iteration over smpt array

Iterate over smpt array using its starting address and length
instead of the blind iterations that used data found in the array.

This prevents possible memory accesses outside of the smpt array
boundaries in case software, or manufacturers, misrepresent smpt
array fields.

Fixes: b038e8e3

 ("mtd: spi-nor: parse SFDP Sector Map Parameter Table")
Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>