net: macb: Fix SUBNS increment and increase resolution

commit 7ad342bc58cc5197cd2f12a3c30b3949528c6d83 upstream.

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. This should be the case on all GEM versions
that this PTP driver supports.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>