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

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.

Cc: <stable@vger.kernel.org> # v3.6 v3.7 v3.8
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>