autosuspend code consolidation

This patch (as813) gathers together common code for USB interface
autosuspend/autoresume. 

It also adds some simple checking at the time an autosuspend request
is made, to see whether the request will fail.  This way we don't
add a workqueue entry when it would end up doing nothing.

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

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