usb ethernet gadget: split out network core

Abstract the peripheral side Ethernet-over-USB link layer code from
the all-in-one Ethernet gadget driver into a component that can be
called by various functions, so the various flavors can be split
apart and selectively reused.

A notable difference from the approach taken with the serial link
layer code (beyond talking to NET not TTY) is that because of the
initialization requirements, this only supports one network link.
(And one set of Ethernet link addresses.)

That is, each configuration may have only one instance of a network
function.  This doesn't change behavior; the current code has that
same restriction.  If you want multiple logical links, that can
easily be done using network layer tools.

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