ASoC: wm9712: Use empty struct initializer

{ 0 } only clears the first member of the structure.

The first member of the snd_soc_dapm_update struct is a pointer,
and writing 0 to a pointer results in the following sparse warning:

sound/soc/codecs/wm9712.c:229:47: warning: Using plain integer as NULL pointer

Use the empty struct initializer that clears all the struct members
and fixes the sparse warning.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>