default or selected kernelversion does not have config value CONFIG_ATH6K_LEGACY.
Result is shown for kernelversion 5.4.4

Atheros AR6003 support (non mac80211)

modulename: ath6kl.ko

Linux Kernel Configuration
└─> Device Drivers
└─> Staging drivers
└─> MMC/SD/SDIO card support
└─> Wireless LAN
└─> Atheros AR6003 support (non mac80211)

This module adds support for wireless adapters based on Atheros AR6003 chipset running over SDIO. If you choose to build it as a module, it will be called ath6kl. Pls note that AR6002 and AR6001 are not supported by this driver.

source code: