AVR32: Fix wrong pin setup for USART3

As reported by Gerhard Berghofer:

in "gpio_enable_usart3" the correct pins for USART 3 are PB17 and PB18
instead of PB18 and PB19.

which is obviously correct. There's currently no code that uses
USART3, but custom boards may run into problems.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>