USB: sierra: fix urb and memory leak on disconnect

commit 014333f77c0b71123d6ef7d31a9724e0699c9548 upstream.

The delayed-write queue was never emptied on disconnect, something which
would lead to leaked urbs and transfer buffers if the device is
disconnected before being runtime resumed due to a write.

Fixes: e6929a90 ("USB: support for autosuspend in sierra while
online")

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>