avr32: 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>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>