ath79: D-Link DAP-2680: select QCA9984 firmware

ath79: D-Link DAP-2680: select QCA9984 firmware

The DAP-2680 has a QCA9984 radio [1], but the commit adding support
mistakenly adds the QCA99x0 firmware package. See forum topic [2].

[1] https://wikidevi.wi-cat.ru/D-Link_DAP-2680_rev_A1
[2] https://forum.openwrt.org/t/missing-5ghz-radio-on-dlink-dap-2680/

Fixes: 5b58710fad21 ("ath79: add support for D-Link DAP-2680 A1")
Signed-off-by: Stijn Segers <foss@volatilesystems.org>
Tested-by: Alessandro Fellin <af.registrazioni@gmail.com>
(cherry picked from commit 0dc056eb66e1b3a4a6797bdf91f7362df6ced9c3)