staging: iio: frequency: ad9834: Provide missing description for 'devid'

staging: iio: frequency: ad9834: Provide missing description for 'devid'

Also demote kernel-doc abuses

Fixes the following W=1 kernel build warning(s):

 drivers/staging/iio/frequency/ad9834.c:87: warning: Function parameter or member 'devid' not described in 'ad9834_state'
 drivers/staging/iio/frequency/ad9834.c:93: warning: cannot understand function prototype: 'enum ad9834_supported_device_ids '
 drivers/staging/iio/frequency/ad9834.c:320: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-iio@vger.kernel.org
Cc: linux-staging@lists.linux.dev
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210414181129.1628598-22-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>