USB: add extension of anchor API, usb_unlink_anchored_urbs

This adds the ability to trigger asynchronous unlinks of anchored URBs. This
is needed for error handling in the comntext of completion handlers, which
cannot sleep.

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