vfio/platform: fix module_put call in error flow

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>