powerpc/fsl_msi: add a const qualifier

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

	arch/powerpc/sysdev/fsl_msi.c: In function 'fsl_of_msi_probe':
	arch/powerpc/sysdev/fsl_msi.c:379:11: error: assignment discards 'const' qualifier from pointer target type [-Werror]
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/powerpc/sysdev/fsl_msi.c

@@ -368,7 +368,7 @@ static int __devinit fsl_of_msi_probe(struct platform_device *dev)
 	int err, i, j, irq_index, count;
 	int rc;
 	const u32 *p;
-	struct fsl_msi_feature *features;
+	const struct fsl_msi_feature *features;
 	int len;
 	u32 offset;
 	static const u32 all_avail[] = { 0, NR_MSI_IRQS };