USB: update Kconfig help text for CONFIG_USB_SUSPEND

This patch (as1429) updates the Kconfig help text for
CONFIG_USB_SUSPEND.  The power/level file is now deprecated; we should
tell people to use power/control instead.

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