usb: dwc3: Add a NULL check in dwc3_resume_work()

If extcon property is not present dwc3_resume_work()
dereferences extcon notifier block pointer to get the
extcon device. This results into NULL pointer
dereference. Add NULL check for extcon to skip extcon
related handling.

Change-Id: I0581cf13a6f577ebaf28817c6afe5839b08b2e95
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>