iio: dac: ad5064: Fix a few kerneldoc misdemeanours

iio: dac: ad5064: Fix a few kerneldoc misdemeanours

Misspelling, missing description.

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

 drivers/iio/dac/ad5064.c:71: warning: bad line:                         internal vref.
 drivers/iio/dac/ad5064.c:83: warning: Function parameter or member 'channels' not described in 'ad5064_chip_info'
 drivers/iio/dac/ad5064.c:125: warning: Function parameter or member 'lock' not described in 'ad5064_state'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200716135928.1456727-20-lee.jones@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>