net: Add tftp speed indication

This prints a tftp speed indication after the download completes. This
is the 3.6 MiB/s indicator below.

Tegra2 (SeaBoard) # tftp ...
Using asx0 device
TFTP from server 172.22.72.144; our IP address is 172.22.73.81
Filename '/tftpboot/uImage-user-seaboard-1'.
Load address: 0x408000
Loading: #################################################
         3.6 MiB/s
done

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>