iio: adc: adi-axi-adc: Drop false marking for kernel-doc

This comment block isn't in kernel-doc format so drop the /** marking.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210314164655.408461-7-jic23@kernel.org