hwmon: (powerz) Avoid cacheline sharing for DMA buffer

Depending on the architecture the transfer buffer may share a cacheline
with the following mutex. As the buffer may be used for DMA, that is
problematic.

Use the high-level DMA helpers to make sure that cacheline sharing can
not happen.

Also drop the comment, as the helpers are documentation enough.

https://sashiko.dev/#/message/20260408175814.934BFC19421%40smtp.kernel.org

Fixes: 4381a36 ("hwmon: add POWER-Z driver")
Cc: stable@vger.kernel.org # ca085fa: dma-mapping: add __dma_from_device_group_begin()/end()
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20260408-powerz-cacheline-alias-v1-1-1254891be0dd@weissschuh.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: t-8ch

drivers/hwmon/powerz.c

@@ -6,6 +6,7 @@
 
 #include <linux/completion.h>
 #include <linux/device.h>
+#include <linux/dma-mapping.h>
 #include <linux/hwmon.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
@@ -33,7 +34,9 @@ struct powerz_sensor_data {
 } __packed;
 
 struct powerz_priv {
-	char transfer_buffer[64];	/* first member to satisfy DMA alignment */
+	__dma_from_device_group_begin();
+	char transfer_buffer[64];
+	__dma_from_device_group_end();
 	struct mutex mutex;
 	struct completion completion;
 	struct urb *urb;