mmc: sdhci: do not try to use 3.3V signaling if not supported

mmc: sdhci: do not try to use 3.3V signaling if not supported

For eMMC devices it is valid to only support 1.8V signaling. When
vqmmc is set to a fixed 1.8V regulator the stack tries to set 3.3V
initially and prints the following warning:
   mmc1: Switching to 3.3V signalling voltage failed

Clear the MMC_SIGNAL_VOLTAGE_330 flag in case 3.3V is signaling is
not available. This prevents the stack from even trying to use
3.3V signaling and avoids the above warning.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>