ar71xx: matchup of TPlink WR841N(D)-names

ar71xx: matchup of TPlink WR841N(D)-names

- remove the suffix (N, ND) to indicate that the image is for all
  WR841-models
- some of these models have a "N"-suffix, others have (ND)-suffix,
  but the boards are the same - only difference is the detachable
  antenna on "D"-models
- discussed this idea to remove the suffix in IRC with jow and Borromini

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48767 3c298f89-4303-0410-b956-a3cf2f4a3e73