staging: dgnc: make dgnc_poll_tick static

No one needs to see this global variable, so make it static.

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