tinydtls: use Message Sequence number == 0 is first Client Hello again

The message Sequence number should be set to 0 in the first Client Hello send to the server.

This fixes a regression introduced in:
commit 9a166b6d9372b20684f38f46b470bde39fcb0541
Author: Hauke Mehrtens <hauke@hauke-m.de>
Date:   Sun Mar 31 14:51:11 2013 +0200

    tinydtls: add new function to send the ClientHello

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>