staging: iio: remove specific chrdev for event reading. Get fd from ioctl on buffer.

Change suggested by Arnd Bergmann.

No real reason to have two chrdevs per device. This step merges them into one.
Currently this means that events will only work on devices with buffers. THat will
be remedied shortly.

V2: set name for event attribute groups.  Otherwise they all sorts of fun occurs on
dynamic channel event attribute creation.

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>