sw test tool: Modify lengths of pad fields for ARP

This patch is after Fixing-MAC-addresses patch.

Wireshark may misunderstand a MAC frame (including ARP) whose length is 64 bytes or more, as a frame with the FCS field.
This patch fixes the issue by modifying lengths of pad fields for ARP.

Reported-by: Arne Goetje <arne_goetje@accton.com>
Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>