mlxsw: spectrum_buffers: Fix pool value handling in mlxsw_sp_sb_tc_pool_bind_set

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