staging/usbip: Change vhci_hcd store_attach() device information message to include speed string

Change vhci_hcd store_attach() routine to include speed string in its device
information message. The current call to dev_info() prints out speed number
which is the enum number. Change to call usb_speed_string() to print speed
string in addition to the number.

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