IB/mthca: Fix off-by-one in mthca SRQ creation

All HCAs (not just mem-free) need a spare SRQ entry, so bump srq->max
by 1 in all cases.

Noted by Jack Morgenstein <jackm@mellanox.co.il>

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