ASoC: wm_hubs: Improve single ended line output enable performance

The enable of the single ended line outputs on wm_hubs devices performs
better if the output is enabled prior to starting VMID. Since inactive
outputs are held at VMID anyway there is little cost to doing this for
unused outputs. Add callbacks into wm_hubs and keep track of which outputs
are really active so we can disable them once we're active.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>