IB/mthca: Return actual capacity from create_srq

Have mthca's create_srq method return the actual capacity of the SRQ
that gets created.  Also update comments in <rdma/ib_verbs.h> to
clarify that this is what is expected from ib_create_srq().

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