clk: tegra: tegra124-emc: Fix clock imbalance in emc_set_timing()

After calling clk_prepare_enable(), clk_disable_unprepare() needs
be called when prepare_timing_change() failed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>