wifi: ath12k: add support for setting fixed HE rate/GI/LTF

wifi: ath12k: add support for setting fixed HE rate/GI/LTF

Add support to set fixed HE rate/GI/LTF values using nl80211.
Reuse parts of the existing code path already used for HT/VHT
to implement the new helpers symmetrically, similar to how
HT/VHT is handled.

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>
Link: https://patch.msgid.link/20250701010408.1257201-7-quic_pradeepc@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>