gpio: Fix kernel-doc comments to nested union

Commit 48ec13d3 ("gpio: Properly document parent data union")
is supposed to have fixed a warning from "make htmldocs" regarding
kernel-doc comments to union members.  However, the same warning
still remains [1].

Fix the issue by following the example found in section "Nested
structs/unions" of Documentation/doc-guide/kernel-doc.rst.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 48ec13d3 ("gpio: Properly document parent data union")
Link: https://lore.kernel.org/r/20220606093302.21febee3@canb.auug.org.au/ [1]
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>

include/linux/gpio/driver.h

@@ -167,21 +167,24 @@ struct gpio_irq_chip {
 	 */
 	irq_flow_handler_t parent_handler;
 
-	/**
-	 * @parent_handler_data:
-	 *
-	 * If @per_parent_data is false, @parent_handler_data is a single
-	 * pointer used as the data associated with every parent interrupt.
-	 *
-	 * @parent_handler_data_array:
-	 *
-	 * If @per_parent_data is true, @parent_handler_data_array is
-	 * an array of @num_parents pointers, and is used to associate
-	 * different data for each parent. This cannot be NULL if
-	 * @per_parent_data is true.
-	 */
 	union {
+		/**
+		 * @parent_handler_data:
+		 *
+		 * If @per_parent_data is false, @parent_handler_data is a
+		 * single pointer used as the data associated with every
+		 * parent interrupt.
+		 */
 		void *parent_handler_data;
+
+		/**
+		 * @parent_handler_data_array:
+		 *
+		 * If @per_parent_data is true, @parent_handler_data_array is
+		 * an array of @num_parents pointers, and is used to associate
+		 * different data for each parent. This cannot be NULL if
+		 * @per_parent_data is true.
+		 */
 		void **parent_handler_data_array;
 	};