mt76: mt7615: fix possible deadlock while mt7615_register_ext_phy()

ieee80211_register_hw() is called with rtnl_lock held, and this could be
caused lockdep from a work item that's on a workqueue that is flushed
with the rtnl held.

Move mt7615_register_ext_phy() outside the init_work().

Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>