iio:imu:bmi160: Fix too large a buffer.

The comment implies this device has 3 sensor types, but it only
has an accelerometer and a gyroscope (both 3D).  As such the
buffer does not need to be as long as stated.

Note I've separated this from the following patch which fixes
the alignment for passing to iio_push_to_buffers_with_timestamp()
as they are different issues even if they affect the same line
of code.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Daniel Baluta <daniel.baluta@oss.nxp.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200920112742.170751-5-jic23@kernel.org