iio: adc: at91_adc: Fix 'bad line' warning

All lines should start with ' *'.

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

 drivers/iio/adc/at91_adc.c:160: warning: bad line:                         (Interruptions registers mostly)

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>