[BUGFIX] Fix warnings on felogin / Use correct usergroup-column

PHP Warning: Illegal string offset 'usergroup' is shown when
trying to login to the frontend while currently already being
logged into the backend.

Also the usergroup-column needs to be fetched from
$this->fe_user->usergroup_column.

Change-Id: Ia9772262616e9bd62e0827b0f211b8efe77ea80b
Resolves: #56508
Releases: 6.2
Reviewed-on: https://review.typo3.org/28000
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert