mmc: sdhci: Fix possible spec. violation during voltage switch sequence

With commit 0797e5f1 (mmc: core: Fixup signal voltage switch), voltage
switch sequence for UHS-I cards is broken if used with sdhci driver.
The commit expects the SD clock to be disabled when mmc_set_ios() is
issued but sdhci_do_set_ios() re-enables the SD clock for few cycles
after disabling which is a specification violation during voltage
switch sequence. This failure is observed only for a small group of
cards where they ultimately fall-back into high-speed mode even if
UHS-I modes are supported.

Change-Id: Ie275326627a84bfcd4352637a043296c01c175a6
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>