ath10k: free beacon buf later in vdev teardown

My wave-1 firmware often crashes when I am bringing down
AP vdevs, and sometimes at least some machines lockup hard
after spewing IOMMU errors.

I don't see the same issue in STA mode, so I suspect beacons
are the issue.

Moving the beacon buf deletion to later in the vdev teardown
logic appears to help this problem.  Firmware still crashes
often, but several iterations did not show IOMMU errors and
machine didn't hang.

Tested hardware: QCA9880
Tested firmware: ath10k-ct from beginning of 2019, exact version unknown

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>