api/spi: Make spi_config parameter constant

As the content of this struct will not be modified by drivers, it's
better to pass it as constant. Also, if someday struct device can be
made contant too, this change will make spi ready for registering the
spi_config into ROM directly.

Fixes #5839

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>