spi/spi-omap2-mcspi: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/spi/spi-omap2-mcspi.c: In function 'omap2_mcspi_probe':
	drivers/spi/spi-omap2-mcspi.c:1118: warning: assignment discards qualifiers from pointer target type
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/spi/spi-omap2-mcspi.c

@@ -1116,7 +1116,7 @@ MODULE_DEVICE_TABLE(of, omap_mcspi_of_match);
 static int __devinit omap2_mcspi_probe(struct platform_device *pdev)
 {
 	struct spi_master	*master;
-	struct omap2_mcspi_platform_config *pdata;
+	const struct omap2_mcspi_platform_config *pdata;
 	struct omap2_mcspi	*mcspi;
 	struct resource		*r;
 	int			status = 0, i;