Move ssl_set_fallback() to work on conf

Initially thought it would be per-connection, but since max_version is in conf
too, and you need to lower that for a fallback connection, the fallback flag
should be in the same place
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index e93182c..f72a2c4 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5569,9 +5569,9 @@
 }
 
 #if defined(MBEDTLS_SSL_FALLBACK_SCSV) && defined(MBEDTLS_SSL_CLI_C)
-void mbedtls_ssl_set_fallback( mbedtls_ssl_context *ssl, char fallback )
+void mbedtls_ssl_set_fallback( mbedtls_ssl_config *conf, char fallback )
 {
-    ssl->fallback = fallback;
+    conf->fallback = fallback;
 }
 #endif