usb: cdns3: Add static to cdns3_gadget_exit function

Function cdns3_gadget_exit is used only in gadget.c file.
This patch removes declaration and definition of this
function from gadget-export.h file and makes it static.

Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>