sysfs: registration_time: use ktime_get_ts

A fix to 03b3ce1a107c4926038931b38894290c074d9e5f: use ktime_get_ts
instead of getnstimeofday to better handle system time changes.
(Shaun Ruffell)

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>