vfio: ccw: process ssch with interrupts disabled

When we call ssch, an interrupt might already be pending once we
return from the START SUBCHANNEL instruction. Therefore we need to
make sure interrupts are disabled while holding the subchannel lock
until after we're done with our processing.

Cc: stable@vger.kernel.org #v4.12+
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>