ar71xx-generic: remove broken mark for wr842n-v3 (#859)

I've bought a couple of those devices from Senetic GmbH.

https://www.senetic.de/product/TL-WR842N



They have 16 MB of Flash and 64 MB of RAM. Platform support works fine,
I've also tested a little with Ethernet (since I saw some regressions on
OpenWRT/LEDE with 841v11), no problems.

Therefore, lets remove the broken mark.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>