usb: dwc3: set 'mode' based on selected Kconfig choices

Now that machines may select dwc3's working mode (HOST only,
GADGET only or DUAL_ROLE) via Kconfig, let's set dwc3's mode
based on that, rather than fixing it to whatever hardware
says.

This way we can skip initializing Gadget/Host in case
we are using Host-only/Gadget-only mode respectively.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>