s390x/sclpconsole: Avoid hanging SCLP ASCII console

Force recalculation of file descriptor sets for main loop's poll(),
in order to be able to readd a possibly removed input file descriptor
after can_read() returned 0 (zero).

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
