imx8mq-mipi-csi2: correctly calculate the hs_settle bits

we currently (in the mainline mipi driver) use the examples that NXP shows for the hs_settle value: https://community.nxp.com/t5/i-MX-Processors/Explenation-for-HS-SETTLE-parameter-in-MIPI-CSI-D-PHY-registers/m-p/764275/highlight/true#M118744 but would like to calculate the correct value for any given data rate.

see also the discussion at https://lore.kernel.org/linux-media/YPCuFA+utjudv11H@pendragon.ideasonboard.com/T/#u