usb: dwc2: gadget: decouple setting soft-disconnect from s3c_hsotg_core_init

This patch changes s3c_hsotg_core_init function to leave hardware in
soft disconnect mode, so the moment of coupling the hardware to the usb
bus can be later controlled by the separate functions for enabling and
disabling soft disconnect mode. This patch is a preparation to rework
pullup() method.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>