usb: ci_udc: use var name ep/ci_ep consistently

Almost all of ci_udc.c uses variable name "ep" for a struct usb_ep and
"ci_ep" for a struct ci_ep. This is nice and consistent, and helps people
know what type a variable is without searching for the declaration.
handle_ep_complete() doesn't do this, so fix it to be consistent.

Signed-off-by: Stephen Warren <swarren@nvidia.com>