gpio: mxc: Unlock on error path in mxc_flip_edge()

We recently added locking to this function but one error path was
over looked.  Drop the lock before returning.

Fixes: e5464277625c ("gpio: mxc: Protect GPIO irqchip RMW with bgpio spinlock")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpio-mxc.c

@@ -249,10 +249,11 @@ static void mxc_flip_edge(struct mxc_gpio_port *port, u32 gpio)
 	} else {
 		pr_err("mxc: invalid configuration for GPIO %d: %x\n",
 		       gpio, edge);
-		return;
+		goto unlock;
 	}
 	writel(val | (edge << (bit << 1)), reg);
 
+unlock:
 	raw_spin_unlock_irqrestore(&port->gc.bgpio_lock, flags);
 }