drivers: usb: udc: Reduce unnecessary ep config lookups

UDC API passes struct udc_ep_config to all functions. Some UDC functions
were using endpoint config structure while some were using device and
endpoint number. This API inconsistency led to completely unnecessary
endpoint structure lookups. Remove unnecessary lookups by using endpoint
config structure pointer where it makes sense.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>