s390/ipl: get rid of ipl_ssid and ipl_devno

ipl_ssid and ipl_devno used to be used during ccw boot when diag308 store
was not available. Reuse ipl_block to store those values.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>