can: softing: softing_card_shutdown(): add braces around empty body in an 'if' statement

This patch fixes the following warning when building the kernel with "W=1":

    warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Link: http://lore.kernel.org/r/20201006203748.1750156-3-mkl@pengutronix.de
Fixes: 03fd3cf5a1 ("can: add driver for Softing card")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/softing/softing_main.c

@@ -447,8 +447,9 @@ static void softing_card_shutdown(struct softing *card)
 {
 	int fw_up = 0;
 
-	if (mutex_lock_interruptible(&card->fw.lock))
+	if (mutex_lock_interruptible(&card->fw.lock)) {
 		/* return -ERESTARTSYS */;
+	}
 	fw_up = card->fw.up;
 	card->fw.up = 0;