[S390] Fix gcc 4.6.0 compile warning

With gcc 4.6.0 we get a false compile warning:

 arch/s390/kernel/setup.c: In function 'setup_arch':
 arch/s390/kernel/setup.c:767:3: warning: 'msg' may be used
            uninitialized in this function [-Wuninitialized]
 arch/s390/kernel/setup.c:753:8: note: 'msg' was declared here

This patch makes gcc quiet.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>