ath11k: clear the keys properly via DISABLE_KEY

DISABLE_KEY sets the key_len to 0, firmware will not delete the keys if
key_len is 0. Changing from security mode to open mode will cause mcast
to be still encrypted without vdev restart.

Set the proper key_len for DISABLE_KEY cmd to clear the keys in
firmware.

Tested-on: IPQ6018 hw1.0 AHB WLAN.HK.2.5.0.1-01100-QCAHKSWPL_SILICONZ-1

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Reported-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Karthikeyan Kathirvel <kathirve@codeaurora.org>
[sven@narfation.org: split into separate patches, clean up commit message]
Signed-off-by: Sven Eckelmann <sven@narfation.org>

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211115100441.33771-1-sven@narfation.org