mmc: sdhci-tegra: drop ->get_ro() implementation

The SDHCI core is know properly checking for the state of a WP GPIO,
so there is no longer any need for the sdhci-tegra code to implement
->get_ro() using mmc_gpio_get_ro().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>