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>