mlxsw: pci: Set mbox dma addresses to zero when not used

Some of the opcodes don't use in, out or both mboxes. In such cases, the
mbox address is a reserved field and FW expects it to be zero.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>