mlxsw: spectrum: acl: Don't encode the key again in mlxsw_sp_acl_atcam_12kb_lkey_id_get()

No need to do key encoding again in
mlxsw_sp_acl_atcam_12kb_lkey_id_get(). Instead of that, introduce
a new helper that would just clear unused blocks.

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>