ASoC: sgtl5000: Remove misleading comment

All new designs should use external VDDD according to official
documentation. See ER1 in errata sheet:
http://cache.nxp.com/files/analog/doc/errata/SGTL5000ER.pdf

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>