USB: UHCI: improve comments and logic for root-hub suspend

This patch (as1488) improves the comments and logic in uhci-hcd's
suspend routine.  The existing comments are hard to understand and
don't give a good idea of what's really going on.

The question of whether EGSM (Enter Global Suspend Mode) and RD
(enable Resume Detect interrupts) can be useful when they're not both
set is difficult.  The spec doesn't give any details on how they
interact with system wakeup, although clearly they are meant to be
used together.  To be safe, the patch changes the subroutine so that
neither bit gets set unless they both do.  There shouldn't be any
functional changes from this; only systems that are designed badly or
broken in some way need to avoid using those bits.

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