wifi: ath9k: remove ath9k_platform_data

Completely unused here in favor of Device Tree based setup. The DT code
in here should currently match what is available with platform files.
Any such lapse can always be added.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/20240906195359.6982-4-rosenp@gmail.com