IB/mthca: Fix check of max_send_sge for special QPs

The MLX transport requires two extra gather entries for sends (one for
the header and one for the checksum at the end, as the comment says).
However the code checked that max_recv_sge was not too big, instead of
checking max_send_sge as it should have.  Fix the code to check the
correct condition.

Signed-off-by: Roland Dreier <rolandd@cisco.com>