rx: Tidy up variables in RTT calc

rx: Tidy up variables in RTT calc

We used to do rttp = &thisRtt, and then use rttp and thisRtt to
interchangably refer to the same data. This is just confusing, and
unnecessary. Replace all of the occurences of rttp with &thisRtt.

Take the opportunity to use the Clock_IsZero macro rather than doing
an explicit zero clock check.

Change-Id: If2e0d7b6dcf96f4e46bde083063115d675a4153f
Reviewed-on: http://gerrit.openafs.org/3031
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>