mmc: sdhci-sirf: kill the "clk" member in driver private struct

Actually we can use the "clk" in the struct sdhci_pltfm_host.
With this change we can also kill the private function for get
max clock in this driver.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>