net: sockets: dtls: reset mbedtls session on timed out handshake

According to MbedTLS API documentation, its session must be
reset if mbedtls_ssl_handshake returns timeout error. This
commit resets the session for said return value, and that
allows us to call send() multiple times even if handshake
times out for previous calls.

Fixes #35711

Signed-off-by: Emil Lindqvist <emil@lindq.gr>