gpiolib: cdev: refactor linehandle cleanup into linehandle_free

Consolidate the cleanup of linehandles, currently duplicated in
linehandle_create and linehandle_release, into a helper function
linehandle_free.

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