clk: shmobile: div6: Fix .recalc_rate() using a stale divisor

cpg_div6_clock_set_rate() only programs the new divisor if the clock
isn't stopped. If the clock is stopped, it will update the cached
divisor value only, which will be programmed into the clock registers
when enabling the clock later.

However, cpg_div6_clock_recalc_rate() reads the divisor from the clock
registers instead of using the cached value, leading to an incorrect
result if the clock is currently stopped.

Make cpg_div6_clock_recalc_rate() use the cached value to fix this.

Reported-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>