ath6kl: Defer wiphy and netdev registration till the end of ath6kl_core_init()

This makes the wiphy and initial netdev registration the last step
in dev initialization. Apart from the fact that this looks right,
it can also be used to setup wiphy with the regulatory information
received from firmware after uploading the firmware. Also it fixes
a FIXME in ath6kl_core_init() where mac address is copied into
netdev->dev_addr, ath6kl_interface_add() takes care of this as well.

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