ath10k: make sure to not use invalid beacon pointer

If DMA mapping of next beacon failed it was
possible for next SWBA to access a pointer that
was already unmapped and freed. This could cause
memory corruption.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>