iio:ad5791: Mark transfer buffers as __be32

Fixes the following warnings from sparse:
	drivers/iio/dac/ad5791.c:114:18: warning: incorrect type in assignment (different base types)
	drivers/iio/dac/ad5791.c:114:18:    expected unsigned int [unsigned] [usertype] d32
	drivers/iio/dac/ad5791.c:114:18:    got restricted __be32 [usertype] <noident>
	drivers/iio/dac/ad5791.c:142:21: warning: incorrect type in assignment (different base types)
	drivers/iio/dac/ad5791.c:142:21:    expected unsigned int [unsigned] [usertype] d32
	drivers/iio/dac/ad5791.c:142:21:    got restricted __be32 [usertype] <noident>
	drivers/iio/dac/ad5791.c:144:21: warning: incorrect type in assignment (different base types)
	drivers/iio/dac/ad5791.c:144:21:    expected unsigned int [unsigned] [usertype] d32
	drivers/iio/dac/ad5791.c:144:21:    got restricted __be32 [usertype] <noident>
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32
	drivers/iio/dac/ad5791.c:148:16: warning: cast to restricted __be32

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/dac/ad5791.c

@@ -107,7 +107,7 @@ enum ad5791_supported_device_ids {
 static int ad5791_spi_write(struct spi_device *spi, u8 addr, u32 val)
 {
 	union {
-		u32 d32;
+		__be32 d32;
 		u8 d8[4];
 	} data;
 
@@ -121,7 +121,7 @@ static int ad5791_spi_write(struct spi_device *spi, u8 addr, u32 val)
 static int ad5791_spi_read(struct spi_device *spi, u8 addr, u32 *val)
 {
 	union {
-		u32 d32;
+		__be32 d32;
 		u8 d8[4];
 	} data[3];
 	int ret;