perf/x86/p4: Add format attributes

Steven reported his P4 not booting properly, the missing format
attributes cause a NULL ptr deref. Cure this by adding the
missing format specification.

I took the format description out of the comment near
p4_config_pack*() and hope that comment is still relatively
accurate.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Bruno Prémont <bonbons@linux-vserver.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Lin Ming <ming.m.lin@intel.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1332859842.16159.227.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>