lib: Fix overwriting action after deactivating callback

If one of the callbacks called from fpi_imgdev_deactivate_complete()
was reactivating the device, then we would be overwriting whichever
"action" got set in the callback, leading to
fpi_imgdev_activate_complete() failing as it doesn't handle the "none"
action.

Reset the action before calling the callbacks.
3 jobs for wip/hadess/gentler-verify-stop in 6 minutes and 18 seconds
Status Job ID Name Coverage
  Build
passed #486736
build

00:05:13

 
  Test
passed #486738
flatpak master

00:01:04

failed #486737
flatpak master

00:00:12