usb: dwc2: rename all s3c_* to dwc2_*

this driver has long ago became dwc2.ko with
both peripheral and host roles, there's no point
in keeping the old function names.

Acked-by: John Youn <johnyoun@synopsys.com>
Tested-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>