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>

cpu/at32ap/at32ap700x/gpio.c

@@ -74,8 +74,8 @@ void gpio_enable_usart2(void)
 
 void gpio_enable_usart3(void)
 {
+	gpio_select_periph_B(GPIO_PIN_PB17, 0);
 	gpio_select_periph_B(GPIO_PIN_PB18, 0);
-	gpio_select_periph_B(GPIO_PIN_PB19, 0);
 }
 #endif