vfio/mdev: make create attribute static

The create attribute is not exported, so make it
static to avoid the following sparse warning:

drivers/vfio/mdev/mdev_sysfs.c:77:1: warning: symbol 'mdev_type_attr_create' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>