media: rcar-csi2: Clarify usage of mbps and msps

The helper function to deal with calculating the link speed is designed
in such a way that it returns the correct type bps (bits per second) for
D-PHY and sps (symbols per second) for C-PHY. And for historical reasons
the function kept the name mbps.

This is confusing, fix it by having the function only deal with bps
values as this is the most common use-case and convert bps to sps in the
only function where it is needed to configure the C-PHY.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20250612175904.1126717-2-niklas.soderlund+renesas@ragnatech.se


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>