ASoC: mediatek: mt6358: fix UNINIT problem

Coverity shows decaring variable reg without initializer. When
regmap_read returns an error, reg keeps the value left from earlier
computation. To prevent from the unexpected result in the case, assign
0 to reg.

Signed-off-by: Trevor Wu <trevor.wu@mediatek.com>
Link: https://lore.kernel.org/r/20230307040938.7484-4-trevor.wu@mediatek.com


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