ath9k: Cleanup mgd_prepare_tx callback

Rename the callback from ath9k_chanctx_force_active()
to ath9k_mgd_prepare_tx(). Also, move it to main.c and
group it with the other callbacks.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>