wifi: ath12k: add read variant from SMBIOS for download board data

This is to read variant from SMBIOS such as read from DT, the variant
string will be used to one part of string which used to search board
data from board-2.bin.

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/20230905105637.10230-1-quic_wgong@quicinc.com