usb: dwc3: remove extcon dependency

It is required by the OMAP glue driver, but it already depends
on it. The core driver should not depend on it. This will
allow the use of the PCI glue driver again.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>