IB/umad: Use stack variable 'base' in ib_umad_init_port

This change is not useful by itself, but sets us up for a future change
that allows us to support more than IB_UMAD_MAX_PORTS in a system.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>