ath79-generic: fix WS-AP3705i autoupdater name (#2817)

The device was introduced in #2332 and merged in 102a4b93.

It appears that the autoupdater name wasn't correct and devices therefore don't receive updates.

root@64295-ggw3-20b399bb366f-132:~# lua -e 'print(require("platform_info").get_image_name())'
enterasys-ws-ap3705i