usb: dwc2: Read GNPTXFSIZ when in forced HOST mode.

The documentation for GNPTXFSIZ says that "For host mode, this field
is always valid."  Since we're already switching to host mode for
HPTXFSIZ, let's also read GNPTXFSIZ in host mode.

On an rk3288 SoC, without this change we see this at bootup:
  dwc2 ff580000.usb: gnptxfsiz=00100400
  dwc2 ff580000.usb: 128 invalid for host_nperio_tx_fifo_size. Check HW configuration.

After this change we see:
  dwc2 ff580000.usb: gnptxfsiz=04000400

Signed-off-by: Doug Anderson <dianders@chromium.org>