ASoC: amd: fix uninitialized variable in snd_acp6x_probe()

The "index" is potentially used without being initialized on the error
path.

Fixes: fc329c1d ("ASoC: amd: add platform devices for acp6x pdm driver and dmic driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211130125633.GA24941@kili


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