Pool index has to be converted by get_pool helper to work correctly for
egress pool. In mlxsw the egress pool index starts from 0.

Fixes: 0f433fa0 ("mlxsw: spectrum_buffers: Implement shared buffer configuration")
Signed-off-by: NJiri Pirko <jiri@mellanox.com>
Reviewed-by: NIdo Schimmel <idosch@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>