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 <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dac/ad5791.c

···
 static int ad5791_spi_write(struct spi_device *spi, u8 addr, u32 val)
 {
 	union {
-		u32 d32;
+		__be32 d32;
 		u8 d8[4];
 	} data;
 
···
 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;