staging: dgnc: Fix unsigned value for less than zero

The "un->un_open_count" is unsigned variable, so it cannot
be less than zero.
Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/dgnc_tty.c

@@ -1636,10 +1636,10 @@ static void dgnc_tty_close(struct tty_struct *tty, struct file *file)
 		un->un_open_count = 1;
 	}
 
-	if (--un->un_open_count < 0) {
+	if (un->un_open_count)
+		un->un_open_count--;
+	else
 		APR(("bad serial port open count of %d\n", un->un_open_count));
-		un->un_open_count = 0;
-	}
 
 	ch->ch_open_count--;