iio: fix kernel-doc build errors

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>