vfio-ccw: make vfio_ccw_async_region_ops static

Since vfio_ccw_async_region_ops is not exported and has no reason to be
globally visible make it static to avoid the following sparse warning:
drivers/s390/cio/vfio_ccw_async.c:73:30: warning: symbol 'vfio_ccw_async_region_ops' was not declared. Should it be static?

Fixes: d5afd5d1 ("vfio-ccw: add handling for async channel instructions")
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>