r8152: reduce the number of Tx

Because the Tx has the features of stopping queue and aggregation,
We don't need many tx buffers. Change the tx number from 10 to 4
to reduce the usage of the memory. This could save 16K * 6 bytes
memory.

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