ath79-generic: TP-Link Archer C7 v2: Fix region selection for factory image (#3259)

The Archer C7 v2 needs a different factory image depending on the region
(EU or US); this got lost during the ar71xx -> ath79 migration.

The OpenWrt factory image without a region suffix is only installable on
very old TP-Link firmwares from before the region split was introduced.
The region-specific firmware works for both old and new TP-Link firmwares.

Fixes: 8fdda51f ("ath79-generic: (re)add support for archer-c7-v2 (#2437)")