time: fix bogus comment in timekeeping_get_ns_raw

The whole point of this function is to return a value not touched by
NTP; unfortunately the comment got copied wholesale without adjustment
from the timekeeping_get_ns function above.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>