gpiolib: cdev: refactor lineevent cleanup into lineevent_free

Consolidate the cleanup of lineevents, currently duplicated in
lineevent_create and lineevent_release, into a helper function
lineevent_free.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>