s390/smp: convert cpuhp_setup_state() return code to zero on success

cpuhp_setup_state() returns a state number on CPUHP_AP_ONLINE_DYN if
no error occurred. Therefore convert the return code to zero before
using it as exit code for s390_smp_init().

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