staging/usbip: simplify usbip_dump_usb_device() udev->speed handling

Change usbip_dump_usb_device() to use usb_speed_string() and remove the
code that does switch on udev->speed and builds custom speed strings.

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>