serial: move WARN_ON() in uart_write() to the condition

uart code currently does the following in uart_write() and
uart_flush_buffer():
  if (cond) {
    WARN_ON(1);
    return;
  }

It can be rewritten to more obvious and more readable:
  if (WARN_ON(cond))
    return;

Do so.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230731080244.2698-2-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>