shell_commands/ping6: Fixed bug in calculation of average round trip time

The average rout trip time has been to low in case packets were lost, as lost
packets were treated as if that had an RTT of 0.

sys/shell/commands/sc_icmpv6_echo.c

@@ -129,7 +129,7 @@ static void _print_stats(char *addr_str, int success, int count, uint64_t total_
     printf("--- %s ping statistics ---\n", addr_str);
 
     if (success > 0) {
-        uint32_t avg_rtt = (uint32_t)(sum_rtt / count); /* get average */
+        uint32_t avg_rtt = (uint32_t)(sum_rtt / success); /* get average */
         printf("%d packets transmitted, %d received, %d%% packet loss, time %"
                PRIu32 ".06%" PRIu32 " s\n", count, success,
                (100 - ((success * 100) / count)),