net: ethtool: Fix documentation of ethtool_sprintf()

This function takes a pointer to a pointer, unlike sprintf() which is
passed a plain pointer. Fix up the documentation to make this clear.

Fixes: 7888fe5 ("ethtool: Add common function for filling out strings")
Cc: Alexander Duyck <alexanderduyck@fb.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20231028192511.100001-1-andrew@lunn.ch
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/ethtool.h

@@ -1045,10 +1045,10 @@ static inline int ethtool_mm_frag_size_min_to_add(u32 val_min, u32 *val_add,
 
 /**
  * ethtool_sprintf - Write formatted string to ethtool string data
- * @data: Pointer to start of string to update
+ * @data: Pointer to a pointer to the start of string to update
  * @fmt: Format of string to write
  *
- * Write formatted string to data. Update data to point at start of
+ * Write formatted string to *data. Update *data to point at start of
  * next string.
  */
 extern __printf(2, 3) void ethtool_sprintf(u8 **data, const char *fmt, ...);