usb: gadget: r8a66597-udc: fix for udc-newstyle

The udc-newstyle needs device_register in probe() of platform_device.
If it doesn't call, kernel panic happens in the sysfs_create_dir() when
we run modprobe a gadget driver.

[ balbi@ti.com : fix compile warning introduced by this patch ]

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>