staging:iio:adis16080: be16 cleanups

The sample buffer contains big endian 16bit words. So use the be16 datatype for
the buffer and use the proper helper functions for endianness conversion instead
of openconding it.

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