wifi: ath12k: convert struct ath12k::wmi_mgmt_tx_work to struct wiphy_work

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.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/20241126171139.2350704-2-kvalo@kernel.org