tmf: Fix unsafe cast warning in TmfTrace

tmf: Fix unsafe cast warning in TmfTrace

Now that TmfData/EventProvider are merged, .getType() will always
return an instance of an ÌTmfEvent. Simply fix the generic types
to take this in consideration.

Change-Id: I791b59c480311a34c7416564219f6d3fed3f5951
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/17054
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Hudson CI