staging: dgnc: add dgnc digi driver

This patch adds the DGNC driver. This is a TTY Serial Port
Driver for the Digi International Neo and Classic PCI based product
line by Digi International <http://www.digi.com

>.

This driver isn't hooked up to the build system because it doesn't
build, it merely adds the driver written by Digi to the kernel tree so
that it can be cleaned up and fixed up properly over time.

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