Hardware experiments show that the negative return value is not just "-EPROBE_DEFER".

How much will this specific error code influence our understanding
of the discussed software situation?

quoted

quoted

+        ret = udc_controller->irq ? : -ENODEV;

Will it be clearer to specify values for all cases in such a conditional operator
(instead of leaving one case empty)?

I don't know what you mean of "instead of leaving one case empty".

I suggest to reconsider also the proposed specification “… ? : …”.

Regards,
Markus