iio: fix kernel-doc build errors

[ Upstream commit c175cb7cd953782bbf4e8bdf088ad61440d6dde5 ]

Fix build errors in kernel-doc notation. Symbols that end in '_'
have a special meaning, but adding a '*' makes them OK.

../drivers/iio/industrialio-core.c:635: ERROR: Unknown target name: "iio_val".
../drivers/iio/industrialio-core.c:642: ERROR: Unknown target name: "iio_val".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>