iio: adc: ina2xx: Fix incorrect report of data endianness to userspace.

This was extracted from a reposting of the driver after it had been applied
to the IIO tree.  I have fast tracked it as the driver will be in 4.5 and
it would be nice to fix this trivial issue before it is.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>