ar71xx: rename firmware for Chinese version of TL-WR941ND v6

ar71xx: rename firmware for Chinese version of TL-WR941ND v6

It seems there are two *different* versions on TL-WR941ND v6: Chinese
and European one. Currently supported by OpenWrt is Chinese one. Rename
firmware to make it clear.
For details see:
http://wiki.openwrt.org/toh/tp-link/tl-wr941nd
https://forum.openwrt.org/viewtopic.php?id=59295

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Backport of r47107

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@48859 3c298f89-4303-0410-b956-a3cf2f4a3e73