net: macb: Fix SUBNS increment and increase resolution

net: macb: Fix SUBNS increment and increase resolution

The subns increment register has 24 bits as follows:
RegBit[15:0] = Subns[23:8]; RegBit[31:24] = Subns[7:0]

Fix the same in the driver and increase sub ns resolution to the
best capable, 24 bits.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>