ASoC: max98090: Fix ill-defined sidetone route

Commit 5fe5b767 ("ASoC: dapm: Do not pretend to support controls for non
mixer/mux widgets") revealed ill-defined control in a route between
"STENL Mux" and DACs in max98090.c:

max98090 i2c-193C9890:00: Control not supported for path STENL Mux -> [NULL] -> DACL
max98090 i2c-193C9890:00: ASoC: no dapm match for STENL Mux --> NULL --> DACL
max98090 i2c-193C9890:00: ASoC: Failed to add route STENL Mux -> NULL -> DACL
max98090 i2c-193C9890:00: Control not supported for path STENL Mux -> [NULL] -> DACR
max98090 i2c-193C9890:00: ASoC: no dapm match for STENL Mux --> NULL --> DACR
max98090 i2c-193C9890:00: ASoC: Failed to add route STENL Mux -> NULL -> DACR

Since there is no control between "STENL Mux" and DACs the control name must
be NULL not "NULL".

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org