IB/mthca: Fix reserved MTTs calculation on mem-free HCAs

The reserved_mtts field has different meaning in Tavor and Arbel, so
we are wasting mtt entries on memfree. Fix the Arbel case to match
Tavor semantics.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>