ASoC: rt5682: remove unwanted btn_type assignment

The following dev_err() is intended to print unexpected btn_type.
Removes the unwanted btn_type assignment.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200430082231.151127-7-tzungbi@google.com
Signed-off-by: Mark Brown <broonie@kernel.org>