ASoC: Fix compile error in sound/soc/mid-x86/sst_platform.c

The variable ret_val is used but not declared. This causes
the following compile error:

sound/soc/mid-x86/sst_platform.c: In function ‘sst_platform_open’:
sound/soc/mid-x86/sst_platform.c:274:2: error: ‘ret_val’ undeclared (first use in this function)
sound/soc/mid-x86/sst_platform.c:274:2: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [sound/soc/mid-x86/sst_platform.o] Error 1

Fix this.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>