ptp_clock: Let the ADJ_OFFSET interface respect the ADJ_NANO flag for PHC devices.

In commit 184ecc9e ("ptp: Add adjphase
function to support phase offset control.") the PTP Hardware Clock
interface expanded to support the ADJ_OFFSET offset mode.  However,
the implementation did not respect the traditional yet pedantic
distinction between units of microseconds and nanoseconds signaled by
the ADJ_NANO flag.  This patch fixes the issue by adding logic to
handle that flag.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Vincent Cheng <vincent.cheng.xh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>