i2c/mpc: add a const qualifier

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

	drivers/i2c/busses/i2c-mpc.c: In function 'fsl_i2c_probe':
	drivers/i2c/busses/i2c-mpc.c:650:31: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/i2c/busses/i2c-mpc.c

@@ -647,7 +647,7 @@ static int __devinit fsl_i2c_probe(struct platform_device *op)
 	}
 
 	if (match->data) {
-		struct mpc_i2c_data *data = match->data;
+		const struct mpc_i2c_data *data = match->data;
 		data->setup(op->dev.of_node, i2c, clock, data->prescaler);
 	} else {
 		/* Backwards compatibility */