usb: serial: propagate errors from ucom_queue_command()

There's only one error that we can get back right now, but future
changes will add some more cases that we need to watch out for. Start
by returning errors and propagating them back.

Reviewed by: kib

(cherry picked from commit 51f3d0874f6216907c8971c4af9d6be6c93edf7b)