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 <[e-mail]>
Cc: Michael Hennerich <[e-mail]>
Cc: Jonathan Cameron <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Acked-by: Jonathan Cameron <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/iio/frequency/ad9834.c

···
  * @spi:		spi_device
  * @mclk:		external master clock
  * @control:		cached control word
+ * @devid:		device id
  * @xfer:		default spi transfer
  * @msg:		default spi message
  * @freq_xfer:		tuning word spi transfer
···
 	__be16				freq_data[2];
 };
 
-/**
+/*
  * ad9834_supported_device_ids:
  */
 
···
 static IIO_DEVICE_ATTR(out_altvoltage0_out1_wavetype_available, 0444,
 		       ad9834_show_out1_wavetype_available, NULL, 0);
 
-/**
+/*
  * see dds.h for further information
  */