IB/mthca: Fix check of size in SRQ creation

The previous patch for Tavor broke MemFree logic.

The driver should perform limit check only for Tavor.  For MemFree,
the check is incorrect, since ds (WQE stride) is always a power-of-2
(although the max_desc_size may not be).

In Tavor, however, WQE stride and desc_size are the same, and are not
necessarily power-of-2.  The check was really for the WQE stride (and
it Tavor, we use max_desc_size for the stride).

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>