coap-server.c: fix typecast warning for verify_cn_callback
coap-server.c: fix typecast warning for verify_cn_callback

This change fixes a warning output by clang for the cast from void* to
the potentially smaller enum coap_dtls_role_t (-Wvoid-pointer-to-enum-cast).

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap