IB/ehca: Fix static rate calculation

The IPD (inter-packet delay) formula was a little off and assumed a
fixed physical link rate; fix the formula and query the actual
physical link rate, now that we can get it.  Also, refactor the
calculation into a common function ehca_calc_ipd() and use that
instead of duplicating code.

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