usb ethernet gadget: split CDC Subset function

This is a simple "CDC Subset" (and MCCI "SAFE") function driver, extracted
from the all-in-one Ethernet gadget driver.

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