ASoC: twl6040: make pointer dmic_codec_dev static

The pointer dmic_codec_dev is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warning:
warning: symbol 'dmic_codec_dev' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>