usb: core: sysfs: Check for SSP rate in speed attr

Check for usb_device->ssp_rate to output the signaling rate for genXxY.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/80a3214592b30da6ca95bb87984f2a9779de0b14.1615432770.git.Thinh.Nguyen@synopsys.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>