wifi: ath12k: fix invalid m3 buffer address

This is to fix m3 buffer reuse issue as m3_mem->size isn't set to
zero in the free function, which leads invalid m3 downloading to
firmware and firmware crashing.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0-03427-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.15378.4

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230721055305.20420-4-quic_wgong@quicinc.com