perf s390: Fix reading cpuid model information

perf s390: Fix reading cpuid model information

Commit 13d7d0eaacd9 (perf record: Provide detailed information on s390
CPU") fixed a  build error on Ubuntu. However the fix uses the wrong
size to print the model information.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Fixes: 13d7d0eaacd9 ("perf record: Provide detailed information on s390 CPU")
Link: http://lkml.kernel.org/r/20180219102444.96900-1-tmricht@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>