net: macb: Add separate definition for PPM fraction

The scaled ppm parameter passed to _adjfine() contains a 16 bit
fraction. This just happens to be the same as SUBNSINCR_SIZE now.
Hence define this separately.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>