mlx4_core: Don't set MTT address in dMPT entries with PA set

If a dMPT entry has the PA flag (direct physical address) set, then
the (unused) MTT base address field has to be set to 0.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>