pcnet: enable the NOUFLO feature

On relatively slow boards (such as the MIPS Malta with an FPGA core
card) it can be extremely common for transmits to underflow - to the
point where it appears they simply do not work at all. Setting the
NOUFLO bit causes the ethernet controller to not begin transmission on
the wire until a transmit start point is reached. Setting that transmit
start point to the full packet will cause the controller to only
transmit the packet once it has buffered it entirely thus preventing any
transmit underflows from occuring and allowing the controller to
function on slower boards.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>