mmc: tegra: Add and use tegra_sdhci_get_max_clock()

Implement and use tegra_sdhci_get_max_clock() which returns the true
maximum host clock rate. The issue with tegra_sdhci_get_max_clock() is
that it returns the current clock rate of the host instead of the
maximum one, which can lead to unnecessarily small clock rates.

This differs from the previous implementation of
tegra_sdhci_get_max_clock() in that it doesn't divide the result by two.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>