Rename resumption_key and the hardcode len

`resumption_key` is better name.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index 93f58e0..672cb0f 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -2055,7 +2055,7 @@
                     MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( resumption ),
                     ticket_nonce,
                     ticket_nonce_len,
-                    session->key,
+                    session->resumption_key,
                     hash_length );
 
     if( ret != 0 )
@@ -2066,11 +2066,11 @@
         return( ret );
     }
 
-    session->key_len = hash_length;
+    session->resumption_key_len = hash_length;
 
     MBEDTLS_SSL_DEBUG_BUF( 3, "Ticket-resumed PSK",
-                           session->key,
-                           session->key_len );
+                           session->resumption_key,
+                           session->resumption_key_len );
 
     return( 0 );
 }