spi: constify of_device_id array

of_device_id is always used as const.
(See driver.of_match_table and open firmware functions)

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-st-ssc4.c

@@ -482,7 +482,7 @@ static const struct dev_pm_ops spi_st_pm = {
 	SET_RUNTIME_PM_OPS(spi_st_runtime_suspend, spi_st_runtime_resume, NULL)
 };
 
-static struct of_device_id stm_spi_match[] = {
+static const struct of_device_id stm_spi_match[] = {
 	{ .compatible = "st,comms-ssc4-spi", },
 	{},
 };