intel/skylake: Fix uninitialized variable warning

I don't think the warning is valid, because we already verify
that num_channels is 2 or 4 as soon as we enter the function.
Adding the default case makes the compiler happy.

Fixes warning:
src/soc/intel/skylake/nhlt/dmic.c: In function 'nhlt_soc_add_dmic_array':
src/soc/intel/skylake/nhlt/dmic.c💯2: error: 'formats' may be used
uninitialized in this function [-Werror=maybe-uninitialized]
return nhlt_endpoint_add_formats(endp, formats, num_formats);
^

Change-Id: Idc22c8478ff666af8915d780d7553909c3163690
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13021
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/soc/intel/skylake/nhlt/dmic.c

@@ -90,6 +90,8 @@ int nhlt_soc_add_dmic_array(struct nhlt *nhlt, int num_channels)
 		num_formats = ARRAY_SIZE(dmic_4ch_cfg);
 		mic_config.array_type = NHLT_MIC_ARRAY_4CH_L_SHAPED;
 		break;
+	default:
+		return -1;
 	}
 
 	if (nhlt_endpoint_append_config(endp, &mic_config, sizeof(mic_config)))