ASoC: SOF: Fix unused variable warnings

The recent fix for the build fix caused a couple of unused variable
compiler warnings when CONFIG_SND_SOC_SOF_NOCODEC isn't set:
  sound/soc/sof/core.c:263:6: warning: unused variable ‘ret’ [-Wunused-variable]
  sound/soc/sof/core.c:262:28: warning: unused variable ‘machine’ [-Wunused-variable]

Fix them by adding another ifdef.

Fixes: ce38a750 ("ASoC: SOF: core: fix undefined nocodec reference")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>