Add counter length macro

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 3f62713..d2f4361 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -594,7 +594,7 @@
 #define MBEDTLS_PREMASTER_SIZE     sizeof( union mbedtls_ssl_premaster_secret )
 
 /* Length of in_ctr buffer in mbedtls_ssl_session */
-#define MBEDTLS_SSL_IN_CTR_LEN 8
+#define MBEDTLS_SSL_COUNTER_LEN 8
 
 #ifdef __cplusplus
 extern "C" {
@@ -1555,7 +1555,7 @@
     size_t MBEDTLS_PRIVATE(out_buf_len);         /*!< length of output buffer          */
 #endif
 
-    unsigned char MBEDTLS_PRIVATE(cur_out_ctr)[8]; /*!<  Outgoing record sequence  number. */
+    unsigned char MBEDTLS_PRIVATE(cur_out_ctr)[MBEDTLS_SSL_COUNTER_LEN]; /*!<  Outgoing record sequence  number. */
 
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
     uint16_t MBEDTLS_PRIVATE(mtu);               /*!< path mtu, used to fragment outgoing messages */