clk: tegra: Implement disable_unused() of tegra_clk_sdmmc_mux_ops

Implement disable_unused() callback of tegra_clk_sdmmc_mux_ops to fix
imbalanced disabling of the unused MMC clock on Tegra210 Jetson Nano.

Fixes: c592c8a2

 ("clk: tegra: Fix refcounting of gate clocks")
Reported-by: Jon Hunter <jonathanh@nvidia.com> # T210 Nano
Tested-by: Jon Hunter <jonathanh@nvidia.com> # T210 Nano
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210717112742.7196-1-digetx@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>