ASoC: mediatek: mtk-btcvsd: remove useless assignment

cppcheck warning:

sound/soc/mediatek/common/mtk-btcvsd.c:783:34: style: Variable 'avail'
is assigned a value that is never used. [unreadVariable]
 int written_size = count, avail = 0, cur_write_idx, write_size, cont;
                                 ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210311004904.121205-2-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>