The external mute (if it is in use) is handled by a GPIO line. Prepare to
remove the set_hs_extmute callback and replace it with:
hs_extmute_gpio: the GPIO number to use for external mute

When the users of set_hs_extmute has been converted the callback can be removed.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>