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>