usb: dwc3: prevent setting PRTCAP to OTG from debugfs

We don't support PRTCAP == OTG yet, so prevent user from
setting it via debugfs.

Fixes: 41ce1456 ("usb: dwc3: core: make dwc3_set_mode() work properly")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>