ASoC: max98357a: Use DAI input as a dapm widget

Spec does not say anything about DAC called SDMode.
It makes more sense to use DAI input that created automatically by DAPM
and route it to Speaker output.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>