drivers: udc: use explicit casts from void* to the target type

This header file provides the public API of the UDC driver,
however it also contains inline function definitions.
Unfortunately these definitions contain implicit pointer conversions,
which are one of the cases where valid C syntax is invalid C++ syntax,
meaning that any C++ source code that intends to use the UDC API
cannot be compiled error-free (or warning-free, if using -fpermissive).

Signed-off-by: Benedek Kupper <kupper.benedek@gmail.com>