usb: dwc3: dwc3-omap: return PROBE_DEFER if extcon is missing and found in dt

Due to inter dependencies of I2C and extcon by the time dwc3
checks for extcon device its not registered especially in case of J6.
In O5 the vbus regulator save us from getting to this point.
So for tiime being return PROBE_DEFER if extcon is enabled in dt.

Signed-off-by: George Cherian <george.cherian@ti.com>