mmc: omap_hsmmc: split duplicate code to calc_divisor() function

There are two places where the same calculations are done.
Let's split them into a separate function.

In addition, simplify by using the DIV_ROUND_UP kernel macro.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>