mmc: sdhci: Avoid accessing register when clocks are turned off

When we disable the clocks, the SDHCI_CLOCK_CONTROL register is already
updated by sdhci-msm platform driver. So avoid writing the same value
again in sdhci_set_clock() API when we are disabling clock. This also
prevents unclocked access issue.

Change-Id: Id5a89f8832ea439e1e2ab6f91a851963c2ac12c8
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>