usb: dwc3: gadget: only resume USB2 PHY in <=HIGHSPEED

As a micro-power optimization, let's only resume the
USB2 PHY if we're working on <=HIGHSPEED. If we're
gonna work on SUPERSPEED or SUPERSPEED+, there's no
point in resuming the USB2 PHY.

Fixes: 2b0f11df

 ("usb: dwc3: gadget: clear SUSPHY bit before ep cmds")
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>