s390/vfio_ccw: make some symbols static

Make some symbols static to fix sparse warnings like:
drivers/s390/cio/vfio_ccw_ops.c:73:1: warning: symbol 'mdev_type_attr_name' was not declared. Should it be static?

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>