rt2x00usb: Use usb anchor to manage URB

With current driver, it is observed that a URB is not
completed while the USB disconnect is initiated. Due to
that, the URB completion handler is trying to access
the resource which was freed as a part of USB disconnect.
Managing the URBs with anchor will make sure that all
the URBs are handled gracefully before device gets
disconnected.

Signed-off-by: Vishal Thanki <vishalthanki@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>