hte: Fix possible use-after-free in tegra_hte_test_remove()

del_timer() does not wait until the timer handler finishing.
This means that the timer handler may still be running after
the driver's remove function has finished, which would result
in a use-after-free.
Fix it by calling del_timer_sync(), which makes sure the timer
handler has finished.

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