IB/ehca: Use default value for Local CA ACK Delay if FW returns 0

Some firmware versions report a Local CA ACK Delay of 0.  In that
case, return a more sensible default value of 12 (-> 16 msec) instead.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>