ASoC: dapm: fix snd_soc_dapm_new_control() implicit declaration

ASoC: dapm: fix snd_soc_dapm_new_control() implicit declaration

The change fixes the following compilation problem:

  sound/soc/soc-dapm.c: In function 'dapm_kcontrol_data_alloc':
  sound/soc/soc-dapm.c:388:4: error: implicit declaration of function
    'snd_soc_dapm_new_control' [-Werror=implicit-function-declaration]
    data->widget = snd_soc_dapm_new_control(widget->dapm,
    ^

  sound/soc/soc-dapm.c:387:17: warning: assignment makes pointer
    from integer without a cast [enabled by default]
    data->widget = snd_soc_dapm_new_control(widget->dapm,
                 ^
  sound/soc/soc-dapm.c: At top level:
  sound/soc/soc-dapm.c:3269:1: error: conflicting types for
    'snd_soc_dapm_new_control'
  snd_soc_dapm_new_control(struct snd_soc_dapm_context *dapm,
  ^

In addition to the fix add static qualifier to
snd_soc_dapm_new_control() function to silence checkpatch.

Fixes: 02aa78abec ("ASoC: DAPM: Add APIs to create individual DAPM controls.")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>