Fix unreachable code warnings with armc5

Some TLS-only code paths were not protected by an #ifdef and while some
compiler are happy to just silently remove them, armc5 complains:

Warning:  #111-D: statement is unreachable

Let's make armc5 happy.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index e6c829d..8089441 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -969,8 +969,11 @@
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
     if( MBEDTLS_SSL_TRANSPORT_IS_DTLS( ssl->conf->transport ) )
         return( 12 );
+    MBEDTLS_SSL_TRANSPORT_ELSE
 #endif
-    return( 4 );
+#if defined(MBEDTLS_SSL_PROTO_TLS)
+        return( 4 );
+#endif
 }
 
 #if defined(MBEDTLS_SSL_PROTO_DTLS)