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>