cleanup-trace-events.pl: Tighten search for trace event call

The script can get fooled too easily.  For instance, it finds
trace_megasas_io_read_start when looking for trace_megasas_io_read,
and incorrectly concludes that event megasas_io_read is used.

Supply -w to git-grep to tighten the search.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1411476811-24251-2-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>