usb ethernet gadget: split CDC Ethernet function

This is a "CDC Ethernet" (ECM) function driver, extracted from the
all-in-one Ethernet gadget driver.

This is a good example of how to implement interface altsettings.
In fact it's currently the only such example in the gadget stack,
pending addition of OBEX support.

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