IB/mlx4: Correct max_srq_wr returned from mlx4_ib_query_device()

We need to keep a spare entry in the SRQ so that there always is a
next WQE available when posting receives (so that we can tell the
difference between a full queue and an empty queue).  So subtract 1
from the value HW gives us before reporting the limit on SRQ entries
to consumers.

Found by Mellanox QA.

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