./net/net.c - make Microsoft dns servers happy with random_port() numbers

For some reason, (which I can't find any documentation on), if U-Boot
gives a port number higher than 17500 to a Microsoft DNS server, the
server will reply to port 17500, and U-Boot will ignore things (since
that isn't the port it asked the DNS server to reply to).

This fixes that by ensuring the random port number is less than 17500.

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>