mlxsw: spectrum_flower: Fix VLAN modify action support

The driver does not support VLAN push and pop, but only VLAN modify.

Fixes: 73867881 ("drivers: net: use flow action infrastructure")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>