[smsc75xx] Add driver for SMSC/Microchip LAN75xx USB Ethernet NICs

[smsc75xx] Add driver for SMSC/Microchip LAN75xx USB Ethernet NICs

This driver is functional but any downloads via a TCP-based protocol
tend to perform poorly.  The 1Gbps Ethernet line rate is substantially
higher than the 480Mbps (in practice around 280Mbps) provided by USB2,
and the device has only 32kB of internal buffer memory.  Our 256kB TCP
receive window therefore rapidly overflows the RX FIFO, leading to
multiple dropped packets (usually within the same TCP window) and
hence a low overall throughput.

Reducing the TCP window size so that the RX FIFO does not overflow
greatly increases throughput, but is not a general-purpose solution.

Further investigation is required to determine how other OSes
(e.g. Linux) cope with this scenario.  It is possible that
implementing TCP SACK would provide some benefit.

Signed-off-by: Michael Brown <mcb30@ipxe.org>