driver:stmmac: Adjust time stamp increase for 0.465 ns accurate only when Time stamp binary rollover is set.

The synopsys spec says When TSCRLSSR is cleard, the rollover value of
sub-second register is 0x7FFFFFFF(0.465 ns per clock).
Signed-off-by: NSonic Zhang <sonic.zhang@analog.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>