[ALSA] sound/soc/soc-dapm.c: make 4 functions static

Make the following needlessly global functions static:
- dapm_power_widgets()
- dapm_mux_update_power()
- dapm_mixer_update_power()
- dapm_free_widgets()

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>