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