Use magic ETH_ADDR_LEN instead of 6 for Ethernet address length.

ETH_ADDR_LEN is defined in lib/packets.h, valued 6.
Use this macro instead of magic number 6 to represent the length
of eth mac address.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>