ALSA: opl3: Replace with __packed attribute

Replace the old __attribute__((packed)) with the new __packed.
Only cleanup, no functional changes.

Link: https://lore.kernel.org/r/20231025132314.5878-9-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/opl3.h

@@ -229,7 +229,7 @@ struct fm_operator {
 	unsigned char attack_decay;
 	unsigned char sustain_release;
 	unsigned char wave_select;
-} __attribute__((packed));
+} __packed;
 
 /* Instrument data */
 struct fm_instrument {