Replace `start` with `ticket_creation`

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index f855576..d727636 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2537,7 +2537,7 @@
 
 #if defined(MBEDTLS_HAVE_TIME) && defined(MBEDTLS_SSL_SRV_C)
     if (session->endpoint == MBEDTLS_SSL_IS_SERVER) {
-        MBEDTLS_PUT_UINT64_BE((uint64_t) session->start, p, 0);
+        MBEDTLS_PUT_UINT64_BE((uint64_t) session->ticket_creation, p, 0);
         p += 8;
     }
 #endif /* MBEDTLS_HAVE_TIME */
@@ -2616,7 +2616,7 @@
         if (end - p < 8) {
             return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
         }
-        session->start = MBEDTLS_GET_UINT64_BE(p, 0);
+        session->ticket_creation = MBEDTLS_GET_UINT64_BE(p, 0);
         p += 8;
     }
 #endif /* MBEDTLS_HAVE_TIME */