s390/sclp: Consolidate early sclp init calls to sclp_early_detect()

The new function calls the old ones. The sclp_event_mask_early() is removed
and replaced by one invocation of sclp_set_event_mask(0, 0).

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>