Yealink uses two URBs that submit each other. This arrangement
cannot be reliably killed with usb_kill_urb() alone, as there's
a window during which the wrong URB may be killed. The fix is
to introduce a flag.

[dtor@mail.ru: remove spinlock, flag alone should be enough]
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>