This patch removes attempts to clear halts on a control endpoint (think
about it for a minute if you don't see why this is pointless....) and also
adds return-code checks for all places where halts are cleared.

This _should_ be just redundant code, but recent tests suggest that this
is, in fact, not the case.  People should _heavily_ test this patch.  I'm
going to pause here for a while (in the patch stream) until we've got this
sorted out -- initial results on my test setup seem to show some problems
still remain.  Where those problems are (HCD or usb-storage) remains to be
seen.