wcn36xx: Disable 5GHz for wcn3620

wcn3620 can only operate on 2.4GHz band due to RF limitation.
If wcn36xx digital block is associated with an external IRIS
RF module, retrieve the id and disable 5GHz band in case of
wcn3620 id.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>