staging:iio: core. Allow for event chrdev obtaining ioctl if no buffer present.

Logic bug meant the chrdev would fail to open if there was no buffer support
in a driver or in the core. This meant the ioctl to get the event chrdev
would fail and hence events were not available.

V2: change error to -EINVAL to mark as unsuitable for reading rather than
not there.  Both are true depending on how you look at it.

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