mlxsw: core: Make counter index allocated inside the action append

So far, the caller of mlxsw_afa_block_append_counter needed to allocate
counter index by hand. Benefit from the previously introduced resource
infra and counter_index_get/put callbacks, and allocate the counter
index in place where it is needed, inside the action append function.

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