USB: Implement PM FREEZE and PRETHAW

This patch (as884) finally implements the time-saving semantics
possible with the Power Management FREEZE and PRETHAW events.  Their
proper handling requires only that devices be quiesced, with
interrupts and DMA turned off; non-root USB devices don't actually
need to be put in a suspended state.  The patch checks and avoids
doing the suspend call when possible.

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