mmc: sdhci: check voltage range only on regulators aware of voltage value

Some regulators don't report any voltage values, so checking supported
voltage range results in disabling all SDHCI_CAN_VDD_* flags and
registration failure. This patch finally provides a correct fix for the
registration of SDHCI driver with all possible voltage regulators:
dummy, fixed and regulated without using regulator_count_voltages()
hacks.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>