mlxsw: item: Add 8bit item helpers

Item heplers for 8bit values are needed, let's add them.

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