mfd: at91-usart: Constify at91_usart_spi_subdev and at91_usart_serial_subdev

They are never get changed, make them constant.
While at it, fix indent as well.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/at91-usart.c

@@ -15,12 +15,12 @@
 #include <linux/of.h>
 #include <linux/property.h>
 
-static struct mfd_cell at91_usart_spi_subdev = {
+static const struct mfd_cell at91_usart_spi_subdev = {
 	.name = "at91_usart_spi",
 	.of_compatible = "microchip,at91sam9g45-usart-spi",
 };
 
-static struct mfd_cell at91_usart_serial_subdev = {
+static const struct mfd_cell at91_usart_serial_subdev = {
 	.name = "atmel_usart_serial",
 	.of_compatible = "atmel,at91rm9200-usart-serial",
 };