hw/timer/exynos4210_mct: Avoid overflow in exynos4210_ltick_recalc_count

Add casts to avoid potentially overflowing the multiplications
of 32 bit quantities in exynos4210_ltick_recalc_count().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
