mmc_test: use ktime_get_ts64 for timestamps

Calling getnstimeofday() can suffer from time jumps and from the
y2038 overflow, so it is not appropriate here. Using ktime_get_ts64()
solves both problems. Using ktime_get() with ktime_t timestamps would
also work, but it seems that we mainly want to print the times as
seconds+nanoseconds, so it would require an extra division in the output.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>