ASoC: Fix early event callback list iteration

The power_list field is used when adding a widget to a power sequence list. Use
the same field when iterating the list using list_for_each_entry, otherwise
we'll see undefined behavior.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>