ASoC: soc-dapm: Use empty struct initializer

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

Even though the first member of the snd_soc_dapm_update struct is a
pointer,it is more robust to use the empty struct initializer that
clears all the struct members.

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