staging: vboxvideo: Add vbox_bo_k[un]map helper functions

Add vbox_bo_k[un]map helper functions instead of having code directly
poking struct vbox_bo internals.

While touch neighboring code anyways also fix a return -PTR_ERR(info),
which should be return PTR_ERR(info).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>