USB: UHCI: Use ACCESS_ONCE rather than using a full compiler barrier

This patch (as1462) updates the special accessor functions defined in
uhci-hcd.h.  Rather than using a full compiler barrier, all we really
need is the ACCESS_ONCE() mechanism, because the idea is to force the
compiler to store a fixed copy of a possibly changing value.

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