IB/ehca: Fix static rate if path faster than link

The formula would yield -1 if the path is faster than the link, which
is wrong in a bad way (max throttling).  Clamp to 0, which is the
correct value.

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