ASoC: cs4271: Remove outdated comment

Commit 1b1861ea ("ASoC: cs4271: convert to direct regmap API usage") removed
the bus_type field from the cs4271_private struct, but left the comment that
described the field in there. This patch removes the comment.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Mark Brown <broonie@linaro.org>