When voltage-ranges property is not present the driver assumes that
it is 3.3v (3.2v..3.4v). But at the same time it disallows polling.

Fix that by dropping the comparison to 0 when no property is provided.

While at it, mark voltage-ranges property optional as it was initially.

Fixes: 9c43df57 ("mmc_spi: Add support for OpenFirmware bindings")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210419112459.25241-3-andriy.shevchenko@linux.intel.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>