ath6kl: Use the other variant of netdev (un)register APIs

Use replace (un)register_netdev() with (un)register_netdevice()
so that the same ath6kl function can be used with
add_virtual_intf()/del_virtual_intf().

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