USB: usb-storage: implement autosuspend

This patch (as1400) adds runtime-PM support to usb-storage.  It
utilizes the SCSI layer's runtime-PM implementation, so its scope is
limited.  Currently the only effect is that disk-like devices (such as
card readers or flash drives) will be autosuspended if they aren't
mounted and their device files aren't open.  This would apply, for
example, to card readers that don't contain a memory card.

Unfortunately this won't interact very well with the removable-media
polling normally carried out by hal or DeviceKit.  Maybe those
programs can be changed to use a longer polling interval, or maybe the
default autosuspend time for usb-storage should be set to something
below 1 second.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: James Bottomley <James.Bottomley@suse.de>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>