usb: dwc3: Start the core in host mode only if there is no extcon

The current driver starts the core in host mode even if
there is extcon available for that core. This causes
the id_state to be overwritten to ID_GROUND irrespective
of the current extcon state.
Fix this starting the core in host mode by default only
if there is no extcon present.

Change-Id: Idd0885942e5e854f0aafc4a26a6eb27033f00935
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>