ath10k: cleanup ath10k_mac_register() error handling

The logic in the error-handling path of ath10k_mac_register() is
divergent from the logic in ath10k_mac_unregister().  Update the
ath10k_mac_register() error handling logic to align with the
ath10k_mac_unregister() logic.

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