usb: cdns3: gadget: Fix full-speed mode

We need to disable USB3 PHY for full-speed mode else
gadget mode is broken.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/20191007121601.25996-3-rogerq@ti.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>