ath11k: fix invalid m3 buffer address

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

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03003-QCAHSPSWPL_V1_V2_SILICONZ_LITE-2

Signed-off-by: Carl Huang <quic_cjhuang@quicinc.com>
Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220209060012.32478-3-quic_wgong@quicinc.com