vfio/platform: fix module_put call in error flow

[ Upstream commit dc51ff91cf2d1e9a2d941da483602f71d4a51472 ]

The ->parent_module is the one that use in try_module_get. It should
also be the one the we use in module_put during vfio_platform_open().

Fixes: 32a2d71c ("vfio: platform: introduce vfio-platform-base module")
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Message-Id: <20210518192133.59195-1-mgurtovoy@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>