UHCI: Add some WARN_ON()s

This patch (as872) adds some WARN_ON()s to various error checks which
are never supposed to fail.  Unsettlingly, one of them has shown up in
a user's log!  Maybe making the warning more visible and having the
call-stack information available will help pinpoint the source of the
problem.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>