usb: dwc2: gadget: use soft-disconnect udc feature in pullup() method

This patch moves udc initialization from pullup() method to
s3c_hsotg_udc_start(), so that method ends with hardware fully
initialized and left in soft-disconnected state. After this change, the
pullup() method simply clears soft-disconnect start() when called with
is_on=1. For completeness, a call to s3c_hsotg_core_disconnect() has
been added when pullup() method is called with is_on=0, what puts the
udc hardware back to soft-disconnected state.

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