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: Shuah Khan <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/usbip/vhci_sysfs.c

···
 	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;
 	}