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

The WRITE_PROTECT bit is always in a "protected mode" on Tegra and
WP-GPIO state need to be used instead. In a case of the GPIO absence,
write-enable should be assumed. External SD is writable once again as
a result of this patch because the offending commit changed behaviour for
the case of a missing WP-GPIO to fall back to WRITE_PROTECT bit-checking,
which is incorrect for Tegra.

Cc: stable@vger.kernel.org # v5.1+
Fixes: e8391453 ("mmc: sdhci-tegra: drop ->get_ro() implementation")
Signed-off-by: Dmitry Osipenko <digetx@gmail.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>