[ALSA] sound/core/pcm.c: make snd_pcm_format_name() static

Modules: PCM Midlevel

This patch makes the needlessly global snd_pcm_format_name() static.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>