tftp: adjust settings to be suitable for 100Mbit ethernet

Adjust timouts and retry counts to be suitable for loaded ethernet
network. With 5 seconds timeout, 10 retries maximum, tftp is
impossible even on local network with single full-speed TCP
connection.

100msec timeout should be suitable for most networks tftp is used on,
that is local ethernets. Timeout count really needs to be way higher,
as lost packets are normal when TCP is running over the same network.

Enforce 10msec minimum.

Signed-off-by: Pavel Machek <pavel@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>