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: ae065d0c ("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>