staging:iio:imu:adis16400 avoid allocating rx, tx, and state separately from iio_dev.

Uses the iio_allocate_device parameter to set aside space for adis16400_state and
____cacheline_aligned buffers for tx and rx to avoid separatel allocating them.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>