Fix memset invocation in rx/event-t.c

The order of the parameters was swapped, which recent gcc complains
loudly about.

Change-Id: I2329ca3dd0eee81639731e78172621b580199024
Reviewed-on: http://gerrit.openafs.org/11451
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>