p54usb: fix leak at failure path in p54u_load_firmware()

If request_firmware_nowait() fails in p54u_load_firmware(),
p54u_load_firmware_cb is not called and no one decrements usb_dev refcnt.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/p54usb.c

@@ -979,6 +979,7 @@ static int p54u_load_firmware(struct ieee80211_hw *dev,
 	if (err) {
 		dev_err(&priv->udev->dev, "(p54usb) cannot load firmware %s "
 					  "(%d)!\n", p54u_fwlist[i].fw, err);
+		usb_put_dev(udev);
 	}
 
 	return err;