[BUGFIX] getAvailableWorkspaces called statically

Use same call to getAvailableWorkspaces as in
checkAccess.

Resolves: #51420
Releases: 6.3, 6.2
Change-Id: Icbea4611d7bd498e25bf658d7f3fa5a74539f479
Reviewed-on: http://review.typo3.org/32562
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>