mlxsw: core_acl_flex_actions: Fix helper to get the first KVD linear index

The helper should return always KVD linear index of the second set.
It is unused now, but going to be used soon.

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>