staging: dgnc: delete the driver

Digi does not support it, no one has hardware for it, and no one is
working on it, so let's drop it for now.  If anyone wants to pick it
back up, then can revert this patch.

Reported-by: Lidza Louina <lidza.louina@gmail.com>
Reported-by: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>