Revert "lib: special handling of dongle disconnect for OS != win32"

This reverts commit f1803fe10f5017e5bf3528d69229d37472a024c8.

Commit f1803fe1 was an early attempt to fix libusb transfers in the
specific case of a disconnected dongle.  The new approach is more robust
(and handles other cancellation causes), but the reverted change also
introduced a behavioral difference in that non-windows hosts aborted
only on repeated failures of a specific type.

The restored behavior is that any error encountered during an
asynchronous transfer cancels the transfer.

Ref: http://lists.osmocom.org/pipermail/osmocom-sdr/2013-February/000481.html
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
libusb_callback: clarify control flow

Replace conditional inline return with a flag processed later in the
function.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
read_async: handle failure in transfer submit

Ensure cancellation proceeds properly when libusb fails to accept a
transfer submission, either during the initial submission of all
asynchronous transfers or when resubmitting.

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