ALSA: pcm: Make snd_pcm_suspend() local static

snd_pcm_suspend() is no longer called from outside, so let's make it
local static.  Also drop a superfluous NULL check there.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>