ASoC: mediatek: mt8183-da7219: remove forward declaration of headset_init

In headset_init(), it can access card by component->card.  Removes the
forward declaration.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200709122445.1584497-3-tzungbi@google.com
Signed-off-by: Mark Brown <broonie@kernel.org>