move client_auth to handshake

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index f261a6a..87fa395 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1701,7 +1701,7 @@
 #if defined(MBEDTLS_SSL_CLI_C)
     if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
     {
-        if( ssl->client_auth == 0 )
+        if( ssl->handshake->client_auth == 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= skip write certificate" ) );
             ssl->state++;