wifi: ath11k: fix writing to unintended memory region

While initializing spectral, the magic value is getting written to the
invalid memory address leading to random boot-up crash. This occurs
due to the incorrect index increment in ath11k_dbring_fill_magic_value
function. Fix it by replacing the existing logic with memset32 to ensure
there is no invalid memory access.

Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.4.0.1-01838-QCAHKSWPL_SILICONZ-1

Fixes: d3d358ef ("ath11k: add spectral/CFR buffer validation support")
Signed-off-by: P Praneesh <quic_ppranees@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230321052900.16895-1-quic_ppranees@quicinc.com