usb-storage: implement autosuspend

This patch (as930) implements autosuspend for usb-storage.  It is
adapted from a patch by Oliver Neukum.  Autosuspend is allowed except
during LUN scanning, resets, and command execution.

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