mlxsw: spectrum_kvdl: Cosmetic kvdl allocator API change

Currently the return allocated index and err value are multiplexed.
This patch changes the API to decouple the ret value from the allocated
index.

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