usb: serial: allow the open/close sleep to be interruptible

ucom_queue_command will issue commands for open/close, then wait on them
to be finished. In the spirit of playing it safe, allow
ucom_queue_command's wait to be interrupted in case the usb process gets
jammed up waiting on the hardware -- we can at least recover the user
thread that initiated it, even if we can't recover the usb process.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D47951