iio:vcnl4000: Mark transfer buffer as __be16

Fixes the following warnings from sparse:
	drivers/iio/light/vcnl4000.c:88:16: warning: cast to restricted __be16
	drivers/iio/light/vcnl4000.c:88:16: warning: cast to restricted __be16
	drivers/iio/light/vcnl4000.c:88:16: warning: cast to restricted __be16
	drivers/iio/light/vcnl4000.c:88:16: warning: cast to restricted __be16

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

drivers/iio/light/vcnl4000.c

@@ -56,7 +56,7 @@ static int vcnl4000_measure(struct vcnl4000_data *data, u8 req_mask,
 				u8 rdy_mask, u8 data_reg, int *val)
 {
 	int tries = 20;
-	u16 buf;
+	__be16 buf;
 	int ret;
 
 	ret = i2c_smbus_write_byte_data(data->client, VCNL4000_COMMAND,