staging: dgnc: make dgnc_poll_lock static

There's no need for this spinlock to be global, so 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>