Add missing mutex unlock on early return in agilent usb driver

When no matching product id was found in the attach function the driver returned without unlocking the agilent_82357a_hotplug_lock mutex.

Add the unlock call.

This was detected by smatch:
smatch warnings:
drivers/staging/gpib/agilent_82357a/agilent_82357a.c:1381 agilent_82357a_attach() warn: inconsistent returns 'global &agilent_82357a_hotplug_lock'.

---
 drivers/staging/gpib/agilent_82357a/agilent_82357a.c | 1 +
 1 file changed, 1 insertion(+)

index b21eadabc466..c22c6bb30776 100644
@@ -1365,6 +1365,7 @@ static int agilent_82357a_attach(gpib_board_t *board, const gpib_board_config_t
 		break;
 	default:
 		dev_err(&usb_dev->dev, "bug, unhandled product_id in switch?\n");
+		mutex_unlock(&agilent_82357a_hotplug_lock);
 		return -EIO;
 	}
 #ifdef RESET_USB_CONFIG

2.47.1