usb: s3c-otg: Rename s3c_udc_*() functions

The driver is actually for the Designware DWC2 controller.
This patch renames the s3c_ep_*() functions to reflect this.
The function s3c_udc_probe() is a special case and is not
renamed by this patch yet.

Signed-off-by: Marek Vasut <marex@denx.de>