ASoC: codecs: wcd938x: fix incorrect used of portid

Mixer controls have the channel id in reg, which is not same as port id.
port id can be derived from chan_info array.
So fix this, with out this, Its possible that we can corrupt
struct wcd938x_sdw_priv if we access array out of bounds.

Fixes: e8ba1e05 ("ASoC: codecs: wcd938x: add basic controls")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>