staging: dgnc: make find_board_by_major static

Fixes the following sparse warning:
drivers/staging/dgnc/dgnc_tty.c:933:19: warning: symbol 'find_board_by_major' was not declared. Should it be static?
Signed-off-by: Benoit Hiller <benoit.hiller@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/dgnc_tty.c

@@ -930,7 +930,7 @@ void dgnc_wakeup_writes(struct channel_t *ch)
 	spin_unlock_irqrestore(&ch->ch_lock, flags);
 }
 
-struct dgnc_board *find_board_by_major(unsigned int major)
+static struct dgnc_board *find_board_by_major(unsigned int major)
 {
 	int i;