mlxsw: spectrum: Replace hard-coded default VID with a define

Subsequent patches are going to replace the current default VID (1) with
VLAN_N_VID - 1 (4095).

Prepare for this conversion by replacing the hard-coded '1' with a
define.

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