staging: iio: Fix incorrect existence check for a shared event pointer.

A second smatch detected error. First part fixes in a typo
in the comment directly above that I noticed whilst trying
to remember what this code actually does. Second part is
the actual fix.  I'm fairly amazed this one never caused
trouble in testing as it is in one of the most common paths.

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