drm: renesas: rz-du: mipi_dsi: Simplify HSFREQ calculation

drm: renesas: rz-du: mipi_dsi: Simplify HSFREQ calculation

Simplify the high-speed clock frequency (HSFREQ) calculation by removing
the redundant multiplication and division by 8. The updated equation:

    hsfreq = mode->clock * bpp / dsi->lanes;

produces the same result while improving readability and clarity.

Additionally, update the comment to clarify the relationship between HS
clock bit frequency, HS byte clock frequency, and HSFREQ.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20250609225630.502888-3-prabhakar.mahadev-lad.rj@bp.renesas.com