stm32/machine_uart: Allow changing only the baudrate.

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/machine_uart.c

@@ -154,6 +154,12 @@ static void mp_machine_uart_init_helper(machine_uart_obj_t *self, size_t n_args,
         { MP_QSTR_read_buf_len, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 64} }, // legacy
     };
 
+    if (self->is_enabled && n_args == 1 && kw_args->used == 0) {
+        // Only change the baudrate if that's all that is given.
+        uart_set_baudrate(self, mp_obj_get_int(pos_args[0]));
+        return;
+    }
+
     // parse args
     struct {
         mp_arg_val_t baudrate, bits, parity, stop, flow, timeout, timeout_char, rxbuf, read_buf_len;