s390/smp: fix smp_find_processor_id() argument mismatch

For SMP and !SMP smp_find_processor_id() either takes a u16 or
an unsigned int argument. Fix this so both versions take a u16.

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