hwmon: ibmaem: Use ktime_get_ns()

Using the wall clock time for delta time calculations is wrong to
begin with because wall clock time can be set from userspace and NTP.
Such data wants to be based on clock monotonic.

The calculations also are done on a nanosecond basis. Use the
nanoseconds based interface right away.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jean Delvare <jdelvare@suse.de>
Acked-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>