staging: dgnc: remove "vpd" board variable

It was only ever read from the device, and spit out to the kernel log
for no good reason.  So remove it, and the way it was read from the
device, and the now unused "board file" callback.

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