mtd: spi-nor: Move condition to avoid a NULL check

When the controller is not under the SPI-MEM interface it may implement
the optional controller_ops->erase() method.

nor->spimem and nor->controller_ops are mutually exclusive. Move the
nor->controller_ops->erase != NULL check as an 'else if' case to
nor->spimem, in order to avoid the nor->controller_ops != NULL
check.

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>