wifi: ath11k: use plain access for descriptor length

wifi: ath11k: use plain access for descriptor length

The read memory barrier added by commit 6d037a372f81 ("wifi: ath11k: fix
ring-buffer corruption") is enough to guarantee ordering also for plain
descriptor accesses if the length helper is ever inlined so drop the
unnecessary READ_ONCE().

Tested-on: WCN6855 hw2.1 WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.41

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Baochen Qiang <quic_bqiang@quicinc.com>
Link: https://patch.msgid.link/20250604143457.26032-3-johan+linaro@kernel.org
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>