iio: adc: at91-sama5d2_adc: Struct kerneldoc titles need to start with 'struct '

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

 drivers/iio/adc/at91-sama5d2_adc.c:360: warning: cannot understand function prototype: 'struct at91_adc_dma '
 drivers/iio/adc/at91-sama5d2_adc.c:379: warning: cannot understand function prototype: 'struct at91_adc_touch '

Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>