iio:imu:adis16400 fix pressure channel scan type

A single channel in this driver was using the IIO_ST macro.
This does not provide a parameter for setting the endianness of
the channel.  Thus this channel will have been reported as whatever
is the native endianness of the cpu rather than big endian. This
means it would be incorrect on little endian platforms.

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