ASoC: Intel: Skylake: Uninitialized variable in probe_codec()

My static checker complains that if snd_hdac_bus_get_response() returns
-EIO then "res" is uninitialized.  Fix this by initializing it to -1 so
that the error is handled correctly.

Fixes: d8c2dab8 ("ASoC: Intel: Add Skylake HDA audio driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>