rockchip: rk3328: don't implement usb_gadget_handle_interrupts twice

The usb_gadget_handle_interrupts()-function is already implemented by
drivers/usb/gadget/dwc2_udc_otg.c, so we need to avoid defining it
in the evb-rk3328.c board-specific file.

This change fixes the following build error (from buildman):
  drivers/usb/gadget/built-in.o: In function `usb_gadget_handle_interrupts':
  build/../drivers/usb/gadget/dwc2_udc_otg.c:850: multiple definition of `usb_gadget_handle_interrupts'
  board/rockchip/evb_rk3328/built-in.o:build/../board/rockchip/evb_rk3328/evb-rk3328.c:37: first defined here
  make[1]: *** [u-boot] Error 1

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>