ath79-generic: switch Wave2 firmware to -ct

This partially reverts commit 22c47df2.

Devices in ath79-generic like the TP-Link EAP225-Outdoor v1 are really
unstable with the non -ct Wave2 firmware and regulary crash with 100% memory
consumption when only a handful devices are connected via 5 GHz.

closes freifunk-gluon/gluon#2827

(cherry picked from commit 4badb270)