iio:adc:ad7298 make the tx and rx buffers __be16

These buffers are a little interesting in that their
content may have variable endianness, but all but one
element will definitely be big endian.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>