Fix clash in ATMEL_USART_ flags

At the moment ATMEL_USART_{RTS,CTS,CLK} have the values
1, 2 and 3 respectively.  Given these are used in bitmasks,
trying to turn on the CLK line will in fact turn on the
RTS and CTS lines as well.

Change the value of ATMEL_USART_CLK to 4.

Signed-off-by: Ben Nizette <bn@niasdigital.com>
---
--- a/arch/avr32/mach-at32ap/include/mach/board.h
+++ b/arch/avr32/mach-at32ap/include/mach/board.h
@@ -29,7 +29,7 @@ extern struct platform_device *atmel_def
 /* Flags for selecting USART extra pins */
 #define	ATMEL_USART_RTS		0x01
 #define	ATMEL_USART_CTS		0x02
-#define	ATMEL_USART_CLK		0x03
+#define	ATMEL_USART_CLK		0x04
 
 struct atmel_uart_data {
 	short		use_dma_tx;	/* use transmit DMA? */