s390/kexec: fix return code handling

kexec_file_add_ipl_report ignores that ipl_report_finish may fail and
can return an error pointer instead of a valid pointer.
Fix this and simplify by returning NULL in case of an error and let
the only caller handle this case.

Fixes: 99feaa71 ("s390/kexec_file: Create ipl report and pass to next kernel")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>