ath11k: fix mgmt_tx_wmi cmd sent to FW for deleted vdev

In Multi-AP VAP scenario with frequent interface up-down, there is a
chance that ath11k_mgmt_over_wmi_tx_work() will dequeue a skb
corresponding to currently deleted/stopped vdev.

FW will assert on receiving mgmt_tx_wmi cmd for already deleted vdev.
Hence adding validation checks for arvif present on the corresponding
ar before sending mgmt_tx_wmi cmd.

Signed-off-by: Sathishkumar Muruganandam <murugana@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1588049126-1490-2-git-send-email-murugana@codeaurora.org