usb ethernet gadget: use composite gadget framework

Building on the previous patches which took code from this driver and
pakaged it in more-reusable network "function" components, this patch
gets rid of the original code and uses those components instead.

As seen with the other gadget driver conversions, the resulting code
is much easier to understand and (presumably) work with.  In this case
that's especially true, since the Ethernet gadget had grown to handle
three (!) different Ethernet-over-USB protocols.  This modularization
should make it much easier to add a fourth option for the newish CDC
"Ethernet Emulation Model" (or EEM).

Lightly tested, primarily at full speed.

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