async_read: clear events after cancelling transfers

In at least one version of libusb-1.0.0, under at least some
circumstances, it appears that the callback is not reliably invoked when
libusb_cancel_transfer() is called multiple times.  Immediately
processing any cancel event appears to eliminate the problem.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>