wifi: ath11k: move HE MCS mapper to a separate function

Move HE MCS mapper to a separate function and call new function
in ath11k_mac_copy_he_cap().

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.4.0.1-00356-QCAHKSWPL_SILICONZ-1

Signed-off-by: Muna Sinada <quic_msinada@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/1666128501-12364-4-git-send-email-quic_msinada@quicinc.com