ASoC: tas2552: Fix kernel crash caused by wrong kcontrol entry

SOC_DAPM_SINGLE("Playback AMP", ..) should not be under kcontrols. It
causes kernel crash (NULL pointer) when the mixers are listed.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org