ASoC: TLV320AIC23: Unquote NULL from control name

Without this I am getting the following messages at boot on my Trimslice:
   tlv320aic23-codec 2-001a: Control not supported for path LLINEIN -> [NULL] -> Line Input
   tlv320aic23-codec 2-001a: ASoC: no dapm match for LLINEIN --> NULL --> Line Input
   tlv320aic23-codec 2-001a: ASoC: Failed to add route LLINEIN -> NULL -> Line Input
   tlv320aic23-codec 2-001a: Control not supported for path RLINEIN -> [NULL] -> Line Input
   tlv320aic23-codec 2-001a: ASoC: no dapm match for RLINEIN --> NULL --> Line Input
   tlv320aic23-codec 2-001a: ASoC: Failed to add route RLINEIN -> NULL -> Line Input
   tlv320aic23-codec 2-001a: Control not supported for path MICIN -> [NULL] -> Mic Input
   tlv320aic23-codec 2-001a: ASoC: no dapm match for MICIN --> NULL --> Mic Input
   tlv320aic23-codec 2-001a: ASoC: Failed to add route MICIN -> NULL -> Mic Input
   tegra-snd-trimslice sound: tlv320aic23-hifi <-> 70002800.i2s mapping ok

Signed-off-by: Liviu Dudau <liviu@dudau.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/tlv320aic23.c

@@ -174,10 +174,9 @@ static const struct snd_soc_dapm_route tlv320aic23_intercon[] = {
 	{"ROUT", NULL, "Output Mixer"},
 
 	/* Inputs */
-	{"Line Input", "NULL", "LLINEIN"},
-	{"Line Input", "NULL", "RLINEIN"},
-
-	{"Mic Input", "NULL", "MICIN"},
+	{"Line Input", NULL, "LLINEIN"},
+	{"Line Input", NULL, "RLINEIN"},
+	{"Mic Input", NULL, "MICIN"},
 
 	/* input mux */
 	{"Capture Source", "Line", "Line Input"},