usb: dwc3: Allow building USB_DWC3_QCOM without EXTCON

Keep EXTCON support optional, as some platforms do not need it.

Do the same for USB_DWC3_OMAP while we're at it.

Fixes: 3def4031

 ("usb: dwc3: add EXTCON dependency for qcom")
Signed-off-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>