iio:ad5421: Mark transfer buffer as __be32

Fixes the following warnings from sparse:
drivers/iio/dac/ad5421.c:134:25: warning: incorrect type in assignment (different base types)
drivers/iio/dac/ad5421.c:134:25: expected unsigned int [unsigned] [usertype] d32
drivers/iio/dac/ad5421.c:134:25: got restricted __be32 [usertype] <noident>
drivers/iio/dac/ad5421.c:168:25: warning: incorrect type in assignment (different base types)
drivers/iio/dac/ad5421.c:168:25: expected unsigned int [unsigned] [usertype] d32
drivers/iio/dac/ad5421.c:168:25: got restricted __be32 [usertype] <noident>
drivers/iio/dac/ad5421.c:172:23: warning: cast to restricted __be32
drivers/iio/dac/ad5421.c:172:23: warning: cast to restricted __be32
drivers/iio/dac/ad5421.c:172:23: warning: cast to restricted __be32
drivers/iio/dac/ad5421.c:172:23: warning: cast to restricted __be32
drivers/iio/dac/ad5421.c:172:23: warning: cast to restricted __be32
drivers/iio/dac/ad5421.c:172:23: warning: cast to restricted __be32

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

drivers/iio/dac/ad5421.c

···
 	 * transfer buffers to live in their own cache lines.
 	 */
 	union {
-		u32 d32;
+		__be32 d32;
 		u8 d8[4];
 	} data[2] ____cacheline_aligned;
 };