iio: adc: twl4030-madc: Fix misnamed struct attribute

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

 drivers/iio/adc/twl4030-madc.c:170: warning: Function parameter or member 'usb3v1' not described in 'twl4030_madc_data'

Also drop a duplicate description of the lock.

Cc: J Keerthy <j-keerthy@ti.com>
Cc: Mikko Ylinen <mikko.k.ylinen@nokia.com>
Cc: Amit Kucheria <amit.kucheria@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>