usb: gadget: pxa27x: fix gadget->dev registration

Whenever ->udc_start() gets called, gadget driver
has already being bound to the udc controller, which
means that gadget->dev had to be already initialized
and added to driver model.

This patch fixes pxa27x mistake.

Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>