powerpc: cell: Use ktime_get_ns()

Replace the ever recurring:
	ts = ktime_get_ts();
	ns = timespec_to_ns(&ts);
with
	ns = ktime_get_ns();

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>