wifi: ath12k: move HE MCS mapper to a separate function

wifi: ath12k: move HE MCS mapper to a separate function

Refactor the HE MCS mapper functionality in
ath12k_mac_copy_he_cap() into a new function.

This helps improve readability, extensibility and will be used
when adding support for 160 MHz bandwidth in subsequent patches.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.0.1-00029-QCAHKSWPL_SILICONZ-1

Co-developed-by: Muna Sinada <quic_msinada@quicinc.com>
Signed-off-by: Muna Sinada <quic_msinada@quicinc.com>
Signed-off-by: Pradeep Kumar Chitrapu <quic_pradeepc@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://patch.msgid.link/20250701010408.1257201-4-quic_pradeepc@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>