i2c: at91: fix compilation warning

This patch fixes the following warning:
drivers/i2c/busses/i2c-at91.c: In function ‘at91_twi_get_driver_data’:
drivers/i2c/busses/i2c-at91.c:411:3: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>

drivers/i2c/busses/i2c-at91.c

@@ -413,7 +413,7 @@ static struct at91_twi_pdata * __devinit at91_twi_get_driver_data(
 		match = of_match_node(atmel_twi_dt_ids, pdev->dev.of_node);
 		if (!match)
 			return NULL;
-		return match->data;
+		return (struct at91_twi_pdata *)match->data;
 	}
 	return (struct at91_twi_pdata *) platform_get_device_id(pdev)->driver_data;
 }