USB: serial: quatech2: fix sleep-while-atomic in close

The write URB was being killed using the synchronous interface while
holding a spin lock in close().

Simply drop the lock and busy-flag update, something which would have
been taken care of by the completion handler if the URB was in flight.

Fixes: f7a33e60 ("USB: serial: add quatech2 usb to serial driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>