[AVR32] Fix duplicate clock index in at32ap machine code

There's a duplicate clock index between USART0 and USART1 which may be
causing system crashes when USART0 is used.  Change the USART0 index
to '3', indicating the clock that is actually used by USART0.

Signed-off-by: Ben Nizette <ben@niasdigital.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>