mtd: spi-nor: Make sure nor->spimem and nor->controller_ops are mutually exclusive

Expand the spi_nor_check() to make sure that nor->spimem and
nor->controller_ops are mutually exclusive.

Fixes: b35b9a10 ("mtd: spi-nor: Move m25p80 code in spi-nor.c")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>