mmc: sdhci-s3c: Simplify min/max clock calculation

This patch reimplements functions calculating minimum and maximum clock
rates to leverage clock rate cache introduced by previous patches.

In addition, the calculation is simplified to just comparing input
clock rates (max case) or input clock rates divided by maximum divisor
(min case), which is basically what the original code did, but with much
more unnecessary work.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by; Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>