USB: mon: use ktime_get_real_ts64 instead of getnstimeofday64

The two do the same thing, but we want to remove getnstimeofday64()
to have a more consistent interface.

It would be nice to use a monotonic clocksource here rather than
'real' time, but that would break the user interface.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>