USB_GADGET_DUALSPEED was removed by commit 85b8614d ("usb: gadget:
get rid of USB_GADGET_{DUAL,SUPER}SPEED"), but the USB_SNP_UDC_PLAT
symbol still selects it.

Remove the USB_GADGET_DUALSPEED select from USB_SNP_UDC_PLAT.

Discovered with the
https://github.com/ulfalizer/Kconfiglib/blob/master/examples/list_undefined.py
script.
Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>