ath10k: plug memory leak on beacon tx

If beacon tx command failed the wmi command buffer
was not freed. This led to OOM in the long run.
This became aparent when stress testing
multi-BSSID.

Reported-By: Tomasz Skapski <tomasz.skapski@tieto.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>