stdio_uart and stdio_uart_init_full don't play nice with flush

[ekholm]

If using stdio_uart_init_full to initialize stdio without using the macros (PICO_DEFAULT_UART_INSTANCE is unset) then
stdio_uart_out_flush will get stuck in assert loop from printf

Suggest that in stdio_uart.c the function stdio_uart_out_flush is implemented using the uart_instance variable

static void stdio_uart_out_flush(void) {
uart_tx_wait_blocking(uart_instance);
}