net: socket: tls: Fix type of pointer

mbedtls_ssl_get_peer_cid takes size_t, not socklen_t.
c546c1cad1f86977dbf3fb3354469db9cda926aa
changed the type of socklen_t to something that is incompatible with native
offloaded sockets, which caused a compiler error.

Signed-off-by: Michael Zimmermann <michael.zimmermann@sevenlab.de>