[S390] sclp: use register_external_interrupt()

Use register_external_interrupt() instead of register_early_external_interrupt().
The early variant is not necessary since kmalloc works already.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>