staging:iio:imu:adis16400 cleanups

a) remove irq is gpio requirement - No reason this needs to be true.
b) use actual part name in info message rather than adis16400 in all cases.
c) scrap use of IIO_CHAN to simplify move out of staging.

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