rtl8139: unify the struct's name

unify the struct's name of "struct rtl8139_private *np" to "struct rtl8139_private *tp"

most of them like this:
struct rtl8139_private *tp = netdev_priv(dev);

Signed-off-by: Amos Kong <jianjun@zeuux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>