mmc: jz4740: Set clock rate to mmc->f_max rather than JZ_MMC_CLK_RATE

The maximum clock rate can be overridden by DT. The clock rate should
be set to the DT-specified value rather than the constant JZ_MMC_CLK_RATE
when this is done. If the maximum clock rate is not set by DT then
mmc->f_max will be set to JZ_MMC_CLK_RATE.

Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>