ALSA: pcm: use "do {} while (0)" for empty macro

Recent compilers produce a harmless warning for the new pcm_call_notify()
macro when CONFIG_SND_PCM_OSS is disabled:

sound/core/pcm.c: In function 'snd_pcm_free':
sound/core/pcm.c:905:37: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]

This turns the empty macro into a 'do {} while (0)' statement to avoid
the warning.

Fixes: 58f30d650c7f ("ALSA: pcm: Build pcm notifier code conditionally")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm.c

@@ -894,7 +894,7 @@ static void snd_pcm_free_stream(struct snd_pcm_str * pstr)
 			_notify->call(pcm);				\
 	} while (0)
 #else
-#define pcm_call_notify(pcm, call) /* NOP */
+#define pcm_call_notify(pcm, call) do {} while (0)
 #endif
 
 static int snd_pcm_free(struct snd_pcm *pcm)