iio:ad5686: Mark transfer buffer as __be32

Fixes the following warnings from sparse:
drivers/iio/dac/ad5686.c:100:25: warning: incorrect type in assignment (different base types)
drivers/iio/dac/ad5686.c:100:25: expected unsigned int [unsigned] [usertype] d32
drivers/iio/dac/ad5686.c:100:25: got restricted __be32 [usertype] <noident>
drivers/iio/dac/ad5686.c:122:25: warning: incorrect type in assignment (different base types)
drivers/iio/dac/ad5686.c:122:25: expected unsigned int [unsigned] [usertype] d32
drivers/iio/dac/ad5686.c:122:25: got restricted __be32 [usertype] <noident>
drivers/iio/dac/ad5686.c:124:25: warning: incorrect type in assignment (different base types)
drivers/iio/dac/ad5686.c:124:25: expected unsigned int [unsigned] [usertype] d32
drivers/iio/dac/ad5686.c:124:25: got restricted __be32 [usertype] <noident>
drivers/iio/dac/ad5686.c:130:16: warning: cast to restricted __be32
drivers/iio/dac/ad5686.c:130:16: warning: cast to restricted __be32
drivers/iio/dac/ad5686.c:130:16: warning: cast to restricted __be32
drivers/iio/dac/ad5686.c:130:16: warning: cast to restricted __be32
drivers/iio/dac/ad5686.c:130:16: warning: cast to restricted __be32
drivers/iio/dac/ad5686.c:130:16: warning: cast to restricted __be32

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

drivers/iio/dac/ad5686.c

···
 	 */
 
 	union {
-		u32 d32;
+		__be32 d32;
 		u8 d8[4];
 	} data[3] ____cacheline_aligned;
 };