ramips: mt7621: fix Ubiquiti ER-X ports names and MAC addresses

ramips: mt7621: fix Ubiquiti ER-X ports names and MAC addresses

The name of each user port should be eth0..4, instead of lan1..4
and there is no WAN port. Rename them to match the official firmware.
To avoid conflict with the master port (gmac0), rename it to "dsa".

The official firmware assigns MAC address in this way:
eth0 = label mac
eth1 = label mac + 1
...
eth4 = label mac + 4

Since we have switched to DSA, it's possible to use different MAC for each port.

Acked-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>