wifi: ath12k: avoid duplicated vdev stop

ath12k_mac_op_unassign_vif_chanctx() will do vdev stop in
ath12k_mac_monitor_stop(). This ath12k_mac_vdev_stop() will do vdev stop
again, then might trigger firmware crash.

So add judgement to avoid duplicated vdev stop.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Fixes: c9e4e41e ("wifi: ath12k: move peer delete after vdev stop of station for WCN7850")
Signed-off-by: Kang Yang <quic_kangyang@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240429081525.983-3-quic_kangyang@quicinc.com