iio: dac: ad5758: Move and fix-up kerneldoc header and demote unworthy kerneldoc

iio: dac: ad5758: Move and fix-up kerneldoc header and demote unworthy kerneldoc

Kerneldoc headers need to come directly before the function/struct
that they are documenting.  Also fix some missing descriptions and
misspellings.

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

 drivers/iio/dac/ad5758.c:111: warning: Function parameter or member 'reg' not described in 'ad5758_range'
 drivers/iio/dac/ad5758.c:111: warning: Function parameter or member 'min' not described in 'ad5758_range'
 drivers/iio/dac/ad5758.c:111: warning: Function parameter or member 'max' not described in 'ad5758_range'
 drivers/iio/dac/ad5758.c:122: warning: Function parameter or member 'gpio_reset' not described in 'ad5758_state'
 drivers/iio/dac/ad5758.c:122: warning: Function parameter or member 'd32' not described in 'ad5758_state'
 drivers/iio/dac/ad5758.c:137: warning: cannot understand function prototype: 'enum ad5758_output_range '

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>