boards/olimex-msp430-h2618: change UART config

Change UART configuration to match `olimex-msp430-h1611`. This allows
using the [olimex-msp430-arduino-uno][board] breakout board to be used
with the `olimex-msp430-h2618` as well.

[board]: https://github.com/RIOT-OS/RIOT-Open-Hardware/tree/main/olimex-msp430-arduino-uno

boards/olimex-msp430-h2618/doc.txt

@@ -93,19 +93,19 @@ make BOARD=olimex-msp430-h2618 flash
 
 ## Using the shell
 
-stdio is available via the UART interface with `TXD = P3.4`
-(pin 35 on the header) and `RXD = P3.5` (pin 34 on the header) at 115,200 Baud.
+stdio is available via the UART interface with `TXD = P3.6`
+(pin 35 on the header) and `RXD = P3.7` (pin 34 on the header) at 115,200 Baud.
 
 The easiest way is to connect an USB TTL adapter (such as the cheap `cp210x`
 or `ch341` based adapters) as follows:
 
 ```
-TTL adapter    Olimex MSP430-H2618
+TTL adapter    Olimex MSP430-H1611
 -----------    -------------------
 
        GND --- 63 (DV_SS)
-       TXD --- 33 (P3.5)
-       RXD --- 32 (P3.4)
+       TXD --- 35 (P3.7)
+       RXD --- 34 (P3.6)
 ```
 
  */

boards/olimex-msp430-h2618/include/periph_conf.h

@@ -49,17 +49,17 @@ static const msp430_clock_params_t clock_params = {
  */
 #define UART_NUMOF          (1U)
 
-#define UART_BASE           (&USCI_A0)
-#define UART_IE             (IE2)
-#define UART_IF             (IFG2)
+#define UART_BASE           (&USCI_A1)
+#define UART_IE             (UC1IE)
+#define UART_IF             (UC1IFG)
 #define UART_IE_RX_BIT      (1 << 0)
 #define UART_IE_TX_BIT      (1 << 1)
 #define UART_RX_PORT        (&PORT_3)
-#define UART_RX_PIN         (1 << 5)
+#define UART_RX_PIN         (1 << 7)
 #define UART_TX_PORT        (&PORT_3)
-#define UART_TX_PIN         (1 << 4)
-#define UART_RX_ISR         (USCIAB0RX_VECTOR)
-#define UART_TX_ISR         (USCIAB0TX_VECTOR)
+#define UART_TX_PIN         (1 << 6)
+#define UART_RX_ISR         (USCIAB1RX_VECTOR)
+#define UART_TX_ISR         (USCIAB1TX_VECTOR)
 /** @} */
 
 /**