usb: dwc2: Move host-specific core functions into hcd.c

Move host core initialization and host channel routines into hcd.c. This
allows these functions to only be compiled in host-enabled driver
configurations (DRD or host-only).

Tested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>