tinydtls: fix some compile warnings

tinydtls: fix some compile warnings

This fixes multiple compiler warnings.

Change-Id: I403fb09b6b8ffc4bc1ca2cc03c90605d9005f8d5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://gerrit.iotivity.org/gerrit/4823
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Soemin Tjong <stjong@microsoft.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>
Reviewed-by: David Antler <david.a.antler@intel.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>