wifi: ath12k: Use msdu_end to check MCBC

We are seeing a very low TCP throughput testing with some specific
tools. This is because for sub-frames of an AMSDU, MCBC flag in
mpdu_start may be not valid, and as a result those frames would be
dropped by kernel.

Add a new helper to get it from msdu_end. Also clean up original
helper since it is not needed any more.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0-03427-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.15378.4

Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230529081609.34567-1-quic_bqiang@quicinc.com