usb: dwc2: Move functions from header to source

Removed extern specifier from dwc2_host_start(), dwc2_host_disconnect()
and dwc2_host_hub_info() functions. Moved those functions from header
to source. Then make them static.

Signed-off-by: Razmik Karapetyan <razmik@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>