ASoC: tas2781: mark dvc_tlv with __maybe_unused

Since we put dvc_tlv static variable to a header file it's copied to
each module that includes the header. But not all of them are actually
used it.

Fix this W=1 build warning:

include/sound/tas2781-tlv.h:18:35: warning: 'dvc_tlv' defined but not
used [-Wunused-const-variable=]

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403290354.v0StnRpc-lkp@intel.com/
Fixes: ae065d0 ("ALSA: hda/tas2781: remove digital gain kcontrol")
Signed-off-by: Gergo Koteles <soyer@irl.hu>
Message-ID: <0e461545a2a6e9b6152985143e50526322e5f76b.1711665731.git.soyer@irl.hu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/tas2781-tlv.h

@@ -15,7 +15,7 @@
 #ifndef __TAS2781_TLV_H__
 #define __TAS2781_TLV_H__
 
-static const DECLARE_TLV_DB_SCALE(dvc_tlv, -10000, 100, 0);
+static const __maybe_unused DECLARE_TLV_DB_SCALE(dvc_tlv, -10000, 100, 0);
 static const DECLARE_TLV_DB_SCALE(amp_vol_tlv, 1100, 50, 0);
 
 #endif