usb: gadget: renesas_usbhs: bugfix: don't modify platform data

renesas_usbhs has default callback functions and settings.
And it tried overwrite to platform private data
if platform doesn't have them.
So, if renesas_usbhs was compiled as module,
it will be hung-up on 2nd insmod.
This patch fixup it.
Special thanks to Bastian

Reported-by: Bastian Hecht <hechtb@googlemail.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>