staging:iio:ad2s90: Add comment to device state mutex

Fix the checkpatch.pl issue:
"CHECK: struct mutex definition without comment".
Signed-off-by: Victor Colombo <victorcolombo@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s90.c

@@ -22,7 +22,7 @@
 #define AD2S90_MAX_SPI_FREQ_HZ  830000
 
 struct ad2s90_state {
-	struct mutex lock;
+	struct mutex lock; /* lock to protect rx buffer */
 	struct spi_device *sdev;
 	u8 rx[2] ____cacheline_aligned;
 };