ath79: apply qca,tx-gain-buffalo

It seems this was only used by one device in ar71xx. Might as well
reapply it.

Make use of the separate TX gain table for WZR-HP-G450H:
https://github.com/torvalds/linux/commit/0f978bfaf2600040f9826f394eb12e023bef3068

Signed-off-by: Rosen Penev <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/15949
Signed-off-by: Hauke Mehrtens <[email protected]>

target/linux/ath79/dts/ar7242_buffalo_wzr-hp-g450h.dts

@@ -79,6 +79,7 @@
 		qca,no-eeprom;
 		#gpio-cells = <2>;
 		gpio-controller;
+		qca,tx-gain-buffalo;
 	};
 };