cpu/cc26xx_cc13xx: Fix bogus array-bound warning

GCC 12 create a bogus array out of bounds warning as it assumes that
because there is special handling for `uart == 0` and `uart == 1`,
`uart` can indeed be `1`. There is an `assert(uart < UART_NUMOF)` above
that would blow up prior to any out of bounds access.

In any case, optimizing out the special handling of `uart == 1` for
when `UART_NUMOF == 1` likely improves the generated code and fixes
the warning.

    /home/maribu/Repos/software/RIOT/cc2650/cpu/cc26xx_cc13xx/periph/uart.c:88:8: error: array subscript 1 is above array bounds of 'uart_isr_ctx_t[1]' [-Werror=array-bounds]
       88 |     ctx[uart].rx_cb = rx_cb;
          |     ~~~^~~~~~
    /home/maribu/Repos/software/RIOT/cc2650/cpu/cc26xx_cc13xx/periph/uart.c:52:23: note: while referencing 'ctx'
       52 | static uart_isr_ctx_t ctx[UART_NUMOF];
          |                       ^~~
    /home/maribu/Repos/software/RIOT/cc2650/cpu/cc26xx_cc13xx/periph/uart.c:89:8: error: array subscript 1 is above array bounds of 'uart_isr_ctx_t[1]' [-Werror=array-bounds]
       89 |     ctx[uart].arg = arg;
          |     ~~~^~~~~~
    /home/maribu/Repos/software/RIOT/cc2650/cpu/cc26xx_cc13xx/periph/uart.c:52:23: note: while referencing 'ctx'
       52 | static uart_isr_ctx_t ctx[UART_NUMOF];
          |                       ^~~

(cherry picked from commit d6499fa)

cpu/cc26xx_cc13xx/periph/uart.c

@@ -64,7 +64,7 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
     int cts_pin = uart_config[uart].cts_pin;
 #endif
 
-    if (uart == 0) {
+    if ((UART_NUMOF == 1) || (uart == 0)) {
         /* UART0 requires serial domain to be enabled */
         if (!power_is_domain_enabled(POWER_DOMAIN_SERIAL)) {
             power_enable_domain(POWER_DOMAIN_SERIAL);