mt76: mt7915: fix possible deadlock while mt7915_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 mt7915_register_ext_phy() outside the init_work().

Signed-off-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>