rp2/machine_uart: Fix setting of UART LCR parameters.

Prior to this change, setting of UART parameters like parity, stop bits or
data bits did not work correctly.  As suggested by @iabdalkader, adding
__DSB() fixes the problem, making sure that changes to the UART LCR_H
register are seen by the peripheral.

Note: the FIFO is already enabled in the call to uart_init(), so the call
to uart_set_fifo_enabled() is not required, but kept for visibility.

Fixes issue micropython#10976.

ports/rp2/machine_uart.c

@@ -336,7 +336,9 @@ STATIC void machine_uart_init_helper(machine_uart_obj_t *self, size_t n_args, co
 
         uart_init(self->uart, self->baudrate);
         uart_set_format(self->uart, self->bits, self->stop, self->parity);
+        __DSB(); // make sure UARTLCR_H register is written to
         uart_set_fifo_enabled(self->uart, true);
+        __DSB(); // make sure UARTLCR_H register is written to
         gpio_set_function(self->tx, GPIO_FUNC_UART);
         gpio_set_function(self->rx, GPIO_FUNC_UART);
         if (self->invert & UART_INVERT_RX) {