staging:iio: use ida_simple_get and ida_simple_remove + merge ids

Use new simple ida allocation functions to remove some boilerplate
code.

Also, now we only have one chdev per device we don't need to maintain
a separate ida for minor numbers. Just use the devices id.

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