mmc: sdhci-msm: Fix issue with 1.8v switch sequence

This is a porting of the logic of the commit <dc18298fc>.
The SD3.0 voltage switch sequence to 1.8v would involve stopping the
SDCLK before changing the voltage level and with recent changes in 3.10
kernel, the peripheral clocks are also getting disabled instead of
just stopping the clock to the card. This patch makes sure this doesn't
happen by marking the flag card_clock_off in struct mmc_host before
starting the voltage switch sequence and checking it in host controller
driver.

Change-Id: Ic20661b5e3c4d5cd88a2b2132901f306928df4bf
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>