USB: uhci: don't use pseudo negative values

The code in uhci-q.c doesn't have to use pseudo-negative values.  I did
it that way because it was easy and because it would give the expected
output during debugging.  But it doesn't have to work that way.  Here's
another approach.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>