mmc: sdhci: ignore restoring the I/O state if MMC_POWER_OFF

One issue was found on a removable high speed sd card with
runtime pm enabled.
When SD card is unplugged, it keep printing "Switching to 3.3V
signalling voltage failed".
And found below sequence triggers the error.

mmc_rescan
	-> mmc_sd_detect
		-> mmc_power_off  -- mmc->ios.vdd is updated to 0.
	-> mmc_claim_host
		-> sdhci_runtime_resume_host
			-> sdhci_start_signal_voltage_switch
				-> mmc_regulator_set_vqmmc
					-> mmc_ocrbitnum_to_vdd

When mmc_ocrbitnum_to_vdd is called, the mmc->ios.vdd is 0, so it
always return -EINVAL. The signal switch will always fail and
print out warning.
Ignore restoring the I/O state when runtime resume if MMC_POWER_OFF.

Signed-off-by: Zhoujie Wu <zjwu@marvell.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>