staging/usbip: Fix vhci_hcd attach failure error message to be informative

When attach fails due to unsupported and/or invalid bus speed, the message
vhci_hcd prints out doesn't include any useful information as to what caused
the failure. Change the message to be informative and use usb_speed_string()
to get the right speed string from usb common.
Signed-off-by: NShuah Khan <shuah.kh@samsung.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/usbip/vhci_sysfs.c

@@ -149,7 +149,8 @@ static int valid_args(__u32 rhport, enum usb_device_speed speed)
 	case USB_SPEED_WIRELESS:
 		break;
 	default:
-		pr_err("speed %d\n", speed);
+		pr_err("Failed attach request for unsupported USB speed: %s\n",
+			usb_speed_string(speed));
 		return -EINVAL;
 	}