mmc: tegra: fix reporting of base clock frequency

Tegra SDHCI controllers, by default, report a base clock frequency of
208Mhz in SDHCI_CAPABILTIES which may or may not be equal to the actual
base clock frequency.  This is because the clock rate is configured by
the clock controller, which is external to the SD/MMC controller.  Since
the SD/MMC controller has no knowledge of how this clock is configured,
it will simply report the maximum frequency.  While the reported value
can be overridden by setting BASE_CLK_FREQ in VENDOR_CLOCK_CTRL on
Tegra30 and later SoCs, just set CAP_CLOCK_BASE_BROKEN and supply
sdhci_pltfm_clk_get_max_clock(), which simply does a clk_get_rate(),
as the get_max_clock() callback.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>