ARM: 6981/2: mmci: adjust calculation of f_min

The ARM version maximum clock divider is 512 whereas for the ST
variants it's 257. Let's use DIV_ROUND_UP() for both cases so we
can see clearly what's going on here.

[Use DIV_ROUND_UP to clarify elder code]

Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Reviewed-by: Sebastian Rasmussen <sebastian.rasmussen@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>