mlxsw: reg: Adjust definition of enum mlxsw_reg_sfgc_type

Define max which would be needed later on.

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