s390x/sclpconsole-lm: Fix hanging SCLP line mode console

Trigger recalculating sets of file descriptors for the main loop's poll()
in order to make sure a possibly removed FD 0 from the poll() file
descriptor array is re-added. FD 0 is removed from the decriptor array
when the console's can_read() callback returns 0.

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>
