ASoC: dapm: Cosmetic tidy up of snd_soc_dapm_new_control

Move the function snd_soc_dapm_new_control to be next to
snd_soc_dapm_new_controls and add some kernel doc for it.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>