ASoC: tpa6130a2: Fix compiler warning

sound/soc/codecs/tpa6130a2.c: In function 'tpa6130a2_add_controls':
sound/soc/codecs/tpa6130a2.c:342: warning: unused variable 'dapm'

Introduced by commit 39646871a47fd8808c08de0ce7d7ca8393af2805 ("ASoC:
tpa6130a2: Replace DAPM code with direct interface").

The DAPM code has been removed from the driver, but the
dapm struct remained.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>