USB: serial: fix race between unthrottle and completion handler in symbolserial

usb:usbserial:symbolserial: fix race between unthrottle and completion handler

symbol_unthrottle() mustn't resubmit the URB unconditionally
as the URB may still be running.

the same bug as opticon.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>