wifi: ath11k: use union for vaddr and iaddr in target_mem_chunk

wifi: ath11k: use union for vaddr and iaddr in target_mem_chunk

The value of 'ab->hw_params.fixed_mem_region' determines that
only one variable 'vaddr' or 'iaddr' is used in target_mem_chunk.
So use an anonymous union instead, easy to check whether the
memory is set or not.

Tested-on: WCN6855 hw2.1 PCI WLAN.HSP.1.1-04358-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1

Signed-off-by: Miaoqing Pan <quic_miaoqing@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://patch.msgid.link/20240813013028.2708111-1-quic_miaoqing@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>