staging: dgnc: make dgnc_num_boards static

There's no need for the world to know the number of dgnc boards in the
system, so just make it a static variable.

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