mlxsw: spectrum: Add mlxsw_sp.mac_mask

The Spectrum hardware demands that all router interfaces in the system
have the same first 38 resp. 36 bits of MAC address: the former limit
holds on Spectrum, the latter on Spectrum-2. Add a field that refers to
the required prefix mask and initialize in mlxsw_sp1_init() and
mlxsw_sp2_init().

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>