staging:iio:ad799x fix incorrect endianness specification for buffer elements

Due to use of the IIO_ST macro, the endianness element of scan_type was
not being set.  The result is that it will default to the cpu endianness
whereas this driver will fill the buffer with big endian data.  Thus for
little endian machines, userspace will miss interpret the data.

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