cpu/sam0/periph/uart: ensure uart_init() idempotency

cpu/sam0_common/periph/uart.c

@@ -57,6 +57,9 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
         return UART_NODEV;
     }
 
+    /* must disable here first to ensure idempotency */
+    dev(uart)->CTRLA.reg &= ~(SERCOM_USART_CTRLA_ENABLE);
+
     /* configure pins */
     gpio_init(uart_config[uart].rx_pin, GPIO_IN);
     gpio_init_mux(uart_config[uart].rx_pin, uart_config[uart].mux);