ath10k: create ath10k_core_init_features()

It's easier to manage firmware version differences when we configure them in
one place. Rename ath10k_core_init_max_sta_count() to
ath10k_core_init_firmware_features() and start moving most of the firmware
version ("features") handling to that function.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>