usbnet: add missing Kconfig for KC2190 cables

Hmm, I noticed that support for one of the USB host-to-host cables
never got fully merged ... Kconfig wouldn't show it!  Fixed.

Also, changed the CDC Subset default to 'y' so that more of these
cables will work out-of-the-box.

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