s390/module: fix compile warning

Get rid of this one (false positive):
arch/s390/kernel/module.c: In function ‘apply_relocate_add’:
arch/s390/kernel/module.c:404:5: warning: ‘rc’ may be used uninitialized
                                 in this function [-Wmaybe-uninitialized]
arch/s390/kernel/module.c:225:6: note: ‘rc’ was declared here

Play safe and preinitialize rc with an error value, so we see an error
if new users indeed don't initialize it.

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