usb: dwc3: prevent setting PRTCAP to OTG from debugfs

usb: dwc3: prevent setting PRTCAP to OTG from debugfs

BugLink: http://bugs.launchpad.net/bugs/1773233
commit daaecc6541d014dca073473ec8a4120c0babbeb4 upstream.

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

Fixes: 41ce1456e1db ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>