ptp: ocp: Have FPGA fold in ns adjustment for adjtime.

The current implementation of adjtime uses gettime/settime to
perform nanosecond adjustments.  This introduces addtional phase
errors due to delays.

Instead, use the FPGA's ability to just apply the nanosecond
adjustment to the clock directly.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>