r8152: set inter fram gap time depending on speed

Set the maximum inter frame gap time (144ns) for speed 10M/half and
100M/half. It improves the performance for those speeds. And, there
is no effect for the other speeds.

For 10M/half and 100M/half, the fast inter frame gap time let the
device couldn't use the feature of the aggregation effectively,
because the transfer would be completed fastly. Therefore, use the
maximum value to improve the effect of the aggregation. However, you
may not feel the improvement for fast CPUs, because they compensate
for the effect of the aggregation.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>