mlxsw: core_acl_flex_actions: Allow the first set to be dummy

In Spectrum-2, the real action sets are always in KVD linear. The first
set is always empty and contains only pointer to the first real set in
KVD linear. So provide possibility to specify the first set is the dummy
one.

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