ASoC: twl6040: Rename the Earphone Driver event handler

Since the event handler is only used by the Earphone Driver, it is better
to rename it from twl6040_power_mode_event to twl6040_ep_drv_event.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>