IB/umad: Fix declaration of dev_map[]

The current ib_umad code never accesses bits past IB_UMAD_MAX_PORTS in
dev_map[].  We shouldn't declare it to be twice as big.

Pointed-out-by: Roland Dreier <rolandd@cisco.com>

Signed-off-by: Hal Rosenstock <halr@voltaire.com>