IB/mthca: Correct reported SRQ size in MemFree case.

MemFree devices need to reserve one shared receive queue (SRQ) work
request for internal use, so the capacity returned from the create_srq
and query_srq methods should be srq->max - 1.

Signed-off-by: Dotan Barak <dotanb@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>