ASoC: rockchip: i2s: Support mono capture

The Rockchip I2S controller only allows to configure even numbers of
capture channels. It is still possible to capture monophonic audio by
using dual-channel mode and ignoring the 'data' from the second
channel.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>