usb: dwc3: use extcon fwrk to receive connect/disconnect

Modified dwc3-omap to receive connect and disconnect notification using
extcon framework. Also did the necessary cleanups required after
adapting to extcon framework.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>