USB: gadget: composite device-level suspend/resume hooks

Address one open question in the composite gadget framework:
Yes, we should have device-level suspend/resume callbacks
in addition to the function-level ones.  We have at least one
scenario (with gadget zero in OTG test mode) that's awkward
to handle without it.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>