usb gadget: link fixes for network gadget

Change how the Ethernet/RNDIS gadget driver builds:  don't
use separate compilation, since it works poorly when key
parts are library code (with init sections etc).  Instead
be as close as we can to "gcc --combine ...".

This is a bit more complicated than most of the others
because it had to resolve a few symbol collisions.

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