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>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -347,7 +347,7 @@ struct at91_adc_trigger {
 };
 
 /**
- * at91_adc_dma - at91-sama5d2 dma information struct
+ * struct at91_adc_dma - at91-sama5d2 dma information struct
  * @dma_chan:		the dma channel acquired
  * @rx_buf:		dma coherent allocated area
  * @rx_dma_buf:		dma handler for the buffer
@@ -369,7 +369,7 @@ struct at91_adc_dma {
 };
 
 /**
- * at91_adc_touch - at91-sama5d2 touchscreen information struct
+ * struct at91_adc_touch - at91-sama5d2 touchscreen information struct
  * @sample_period_val:		the value for periodic trigger interval
  * @touching:			is the pen touching the screen or not
  * @x_pos:			temporary placeholder for pressure computation