usb: dwc3: core: fix ordering for PHY suspend

We can't suspend the PHYs before dwc3_core_exit_mode()
has been called, that's because the host and/or device
sides might still need to communicate with the far end
link partner.

Fixes: 8ba007a9 (usb: dwc3: core: enable the USB2 and USB3 phy in probe)
Cc: <stable@vger.kernel.org> # v3.9+
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>