When attempting to read the 'output' node, interpret any error *other*
than ENOENT as a fatal error. For ENOENT, fall back to serial_hd() to
find a character device, or create a null device.
Do not attempt to prepend to errp when serial_hd() fails; the error
isn't relevant (and prior to this change, wasn't set anyway).
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>