ASoC: Disable thermal shutdown when not using speakers in wm_hubs

The thermal shutdown support in wm_hubs devices is tied to the speaker
drivers (which are the only high power subsystems within the device).
Ensure minimal current usage when the thermal shutdown support is not
required by disabling the circuit when the speaker drivers are powered
down.

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