ALSA: ppc/awacs: shut up maybe-uninitialized warning

The awacs sound driver produces a false-positive warning in ppc64_defconfig:

sound/ppc/awacs.c: In function 'snd_pmac_awacs_init':
include/sound/control.h:219:9: warning: 'master_vol' may be used uninitialized in this function [-Wmaybe-uninitialized]

I haven't come up with a good way to rewrite the code to avoid the
warning, so here is a bad one: I initialize the variable before
the conditionall initialization so gcc no longer has to worry about
it.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/ppc/awacs.c

@@ -991,6 +991,7 @@ snd_pmac_awacs_init(struct snd_pmac *chip)
 		if (err < 0)
 			return err;
 	}
+	master_vol = NULL;
 	if (pm7500)
 		err = build_mixers(chip,
 				   ARRAY_SIZE(snd_pmac_awacs_mixers_pmac7500),