ALSA: isa/gus: remove 'set but not used' warning

Fix W=1 warning by noting variable as __always_unused.

sound/isa/gus/gus_uart.c: In function ‘snd_gf1_interrupt_midi_in’:
sound/isa/gus/gus_uart.c:16:22: warning: variable ‘data’ set but not
used [-Wunused-but-set-variable]
   16 |  unsigned char stat, data, byte;
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-3-pierre-louis.bossart@linux.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/gus/gus_uart.c

@@ -13,7 +13,8 @@
 static void snd_gf1_interrupt_midi_in(struct snd_gus_card * gus)
 {
 	int count;
-	unsigned char stat, data, byte;
+	unsigned char stat, byte;
+	__always_unused unsigned char data;
 	unsigned long flags;
 
 	count = 10;