USB: serial: propagate late probe errors

Propagate errnos for late probe errors (e.g. -ENOMEM on allocation
failures) instead of always returning -EIO.

Note that some drivers are currently returning -ENODEV from their attach
callbacks when a device is not supported, but this has also been mapped
to -EIO.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>