IB/mlx4: Disable TSO for Connect-X rev. A0 HCAs

According to <http://marc.info/?t=138347640900004&r=1&w=2

>, revision
A0 of Connect-X does not correctly assemble TSO packets.  Disable that
feature on that hardware revision.

Signed-off-by: Roland Dreier <roland@purestorage.com>