diffie-hellman: Use bool instead of status_t as get_shared_secret() return value

While such a change is not unproblematic, keeping status_t makes the API
inconsistent once we introduce return values for the public value operations.

src/charon-tkm/src/tkm/tkm_diffie_hellman.c

@@ -61,11 +61,11 @@ METHOD(diffie_hellman_t, get_my_public_value, void,
 	sequence_to_chunk(this->pubvalue.data, this->pubvalue.size, value);
 }
 
-METHOD(diffie_hellman_t, get_shared_secret, status_t,
+METHOD(diffie_hellman_t, get_shared_secret, bool,
 	private_tkm_diffie_hellman_t *this, chunk_t *secret)
 {
 	*secret = chunk_empty;
-	return SUCCESS;
+	return TRUE;
 }