powerpc/vas: Fix cleanup when VAS is not configured

powerpc/vas: Fix cleanup when VAS is not configured

BugLink: http://bugs.launchpad.net/bugs/1786352
[ Upstream commit 45ddea8a73a25461387eb8e87f3e0ecca084799b ]

When VAS is not configured, unregister the platform driver. Also simplify
cleanup by delaying vas debugfs init until we know VAS is configured.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>