usb: gadget: s3c-hsudc: remove the_controller global

Instead use container_of to retrieve the s3c_hsudc from the
struct usb_gadget pointer.

[ balbi@ti.com : changed verbose container_of() into
	an already provided helper 'to_hsudc()' ]

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