mmc: sdhci: Introduce sdhci_calc_clk()

In order to remove the SDHCI_QUIRK2_NEED_DELAY_AFTER_INT_CLK_RST and to
reduce code duplication, put the code relative to the SD clock
configuration in a function which can be used by hosts for the
implementation of the ->set_clock() callback.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>