clk: renesas: rzv2h: Adjust for CPG_BUS_m_MSTOP starting from m = 1

clk: renesas: rzv2h: Adjust for CPG_BUS_m_MSTOP starting from m = 1

Avoid using the "- 1" for finding mstop_index in all functions accessing
priv->mstop_count, by adjusting its pointer in rzv2h_cpg_probe().

While at it, drop the intermediate local variable index.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Closes: https://lore.kernel.org/all/CAMuHMdX1gPNCFddg_DyK7Bv0BeFLOLi=5eteT_HhMH=Ph2wVvA@mail.gmail.com/
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250222142009.41324-1-biju.das.jz@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>