mlxsw: spectrum_flower: Offload FLOW_ACTION_PRIORITY

Offload action skbedit priority when keyed to a flower classifier. The
skb->priority field in Linux is very generic, so only allow setting the
bottom 8 priorities and bounce anything else.

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