To simplify locking for the next patches convert struct
ath12k::wmi_mgmt_tx_work to use wiphy_work. After this
ath12k_mgmt_over_wmi_tx_work() is called with wiphy_lock() taken. In
ath12k_core_suspend() we need to take wiphy_lock() because
ath12k_mac_wait_tx_complete() requires it.
Also add lockdep_assert_wiphy() to document when wiphy_lock() is held.