USB: address FIXME in usbnet w.r.t drivers claiming multiple interfaces

This fixes the issue of drivers claiming multiple interfaces. Operations
are stopped as soon as an interface is suspend and resumed only as
all interfaces have been resumed.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>