usb: cdns3: Changed type of gadget_dev in cdns structure

Patch changes the type for gadget_dev pointer in cdns structure from
pointer to cdns3_device structure to void pointer.
This filed is in reusable code and after this change it will be used to
point to both cdns3_device or cdnsp_device objects.

Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Tested-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>