ASoC: dapm: Add helper function to free a widget

snd_soc_tplg_widget_remove_all() has a verbatim copy of an older version of
the widget freeing code from dapm_free_widgets(). Add a new helper function
that takes care of freeing a widget and use it in both places.

This removes the duplicated code and also makes sure that future changes to
the widget freeing code only have to be made in one location.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>