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

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.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>