wifi: ath9k: Clear DMA descriptors without memset

wifi: ath9k: Clear DMA descriptors without memset

Clear ath9k DMA descriptors with explicit status word stores instead of
memset(). The descriptor rings are coherent DMA memory, which may be
mapped uncached on 32-bit powerpc. The optimized memset() path can use
dcbz there and trigger an alignment warning.

Use WRITE_ONCE() for the descriptor status words so the compiler keeps
the clears as ordinary stores instead of folding them back into bulk
memset(). This covers AR9003 TX status descriptors as well as the RX
status area cleared when setting up RX descriptors.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20260517042716.2218386-1-rosenp@gmail.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>