[S390] arch/s390/kernel/ipl.c: correct error detection check

reipl_fcp_kset was just initialized, so it appears that it should be tested
instead of reipl_kset.

Signed-off-by: Julia Lawall <julia@diku.dk>
Reported-by: Suman Saha <sumsaha@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>