ASoC: mioa701_wm9713: Fix speaker event

Commit f6b2a045 ("ASoC: pxa: mioa701_wm9713: Convert to table based DAPM
setup") converted the driver to register the board level DAPM elements with
the card's DAPM context rather than the CODEC's DAPM context. The change
overlooked that the speaker widget event callback accesses the widget's
codec field which is only valid if the widget has been registered in a CODEC
DAPM context. This patch modifies the callback to take an alternative route
to get the CODEC.

Fixes: f6b2a045 ("ASoC: pxa: mioa701_wm9713: Convert to table based DAPM
setup")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>