usb: gadget: u_serial: Avoid double unlock of serial_port_lock

Avoid unlocking serial_port_lock twice in gserial_suspend(), this can
occur if gserial_wakeup_host() fails. And since wakeup is performed
outside spinlock, check if the port is valid before proceeding again.

Fixes: 3baea29 ("usb: gadget: u_serial: Implement remote wakeup capability")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/aBHatifO5bjR1yPt@stanley.mountain/
Signed-off-by: Prashanth K <prashanth.k@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250506104912.3750934-1-prashanth.k@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Prashanth K

drivers/usb/gadget/function/u_serial.c

@@ -1505,6 +1505,13 @@ void gserial_suspend(struct gserial *gser)
 		spin_unlock_irqrestore(&serial_port_lock, flags);
 		if (!gserial_wakeup_host(gser))
 			return;
+
+		/* Check if port is valid after acquiring lock back */
+		spin_lock_irqsave(&serial_port_lock, flags);
+		if (!port) {
+			spin_unlock_irqrestore(&serial_port_lock, flags);
+			return;
+		}
 	}
 
 	spin_lock(&port->port_lock);