usb: gadget: s3c-hsudc: move device registration to probe

Instead of adding and deleting the gadget device in the start and stop
invocations. Use device_register in the probe method to initialize
and add the gadget device.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>