ASoC: tlv320aic31xx: do not declare support for mono DAI

This hardware supports only 2-channel DAI, even mono ADC digital output
has two channels with the same data.

Having min_channels=1 results in broken playback of mono files in setups
where CPU DAI supports mono.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Mark Brown <broonie@kernel.org>