This patch creates the first of several separate "minidriver" modules
for "usbnet".  This one handles only the very simplest hardware, which
can be handled almost entirely by the "usbnet" core.

    - Move device-specific bits into new "cdc_subset.c" driver,
      shrinking "usbnet" by a bunch;

    - Export the functions needed to support this minidriver
      (with EXPORT_SYMBOL_GPL);

    - Update Kconfig and kbuild accordingly.

This one handles about a dozen different device types, with the most
notable ones being Gumstix and most Linux-based PDAs (except Zaurus
running that ancient code from Sharp).

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