wifi: ath12k: use plain access for descriptor length

wifi: ath12k: use plain access for descriptor length

The read memory barrier added by commit 6b67d2cf14ea ("wifi: ath12k: 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: WCN7850 hw2.0 WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Baochen Qiang <quic_bqiang@quicinc.com>
Link: https://patch.msgid.link/20250617084402.14475-3-johan+linaro@kernel.org
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>