usb: dwc3: Enable usb2 LPM only when connected as usb2.0

commit 2b758350 upstream.

Synopsys says:
The HIRD Threshold field must be set to ‘0’ when the device core is
operating in super speed mode.

This patch implements above statement.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>