ARM: tegra: acer-a500: Specify proper voltage for WiFi SDIO bus

Tegra20 has v2.00 SDMMC controller which doesn't support voltage
switching and the WiFi SDIO bus voltage is fixed to 1.8v in accordance
to the board's schematics, while MMC core confusingly saying that it's
3.3v because of the v2.00. Let's correct the voltage in the device-tree
just for consistency. This is a minor improvement which doesn't fix any
problems.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>