usb: dwc3: dwc3_get_extcon() can be static

Fix sparse warning

Fixes: 5f0b74e5 ("USB: dwc3: get extcon device by OF graph bindings")
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>