Allows us to drop the cleanup code on the floor.

Sam noticed in his review:
> With this change we avoid calling pci_disable_device()
> twise in case vbox_mm_init() fails.
> Once in vbox_hw_fini() and once in the error path.

v2: Include Sam's review remarks

v3: Fix typo in commit summary (Thomas Zimmermann)
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-6-daniel.vetter@ffwll.ch