The delayed-write queue was never emptied at shutdown (close), something
which could lead to leaked urbs if the port is closed before being
runtime resumed due to a write.
When this happens the output buffer would not drain on close
(closing_wait timeout), and after consecutive opens, writes could be
corrupted with previously buffered data, transfered with reduced
throughput or completely blocked.
Note that unbusy_queued_urb() was simply moved out of CONFIG_PM.
Fixes: 383cedc3bb43 ("USB: serial: full autosuspend support for the
option driver")