ASoC: mediatek: mt2701: fix some error handling in probe

The check for if the "afe" allocation failed was too late and there
wasn't a check for "afe->platform_priv".

Fixes: 43a6a7e7 ('ASoC: mediatek: add mt2701 platform driver implementation.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Garlic Tseng <garlic.tseng@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>