iio: adc: ad7949: Fix misspelling issue

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

 drivers/iio/adc/ad7949.c:58: warning: Function parameter or member 'indio_dev' not described in 'ad7949_adc_chip'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Link: https://lore.kernel.org/r/20200716135928.1456727-18-lee.jones@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>