scsi_debug: fix missing "break;" in SDEBUG_UA_CAPACITY_CHANGED case

This eliminates a superfluous log message when the capacity is changed:

  "check_readiness: unexpected unit attention code=3"

Signed-off-by: Ewan D. Milne <emilne@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>