lan78xx: Set flow control threshold to prevent packet loss

Set threshold at which flow control is triggered to 3/4 full of
the internal Rx packet FIFO to prevent packet drops at high data
rates. The new setting reduces the number of dropped UDP frames
and TCP retransmit requests especially on less capable CPUs.

Signed-off-by: John Efstathiades <john.efstathiades@pebblebay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>