x86, mce: Fix compile warning in case of CONFIG_SMP=n

Fix following compile warning:

  arch/x86/kernel/cpu/mcheck/mce_amd.c: In function 'threshold_create_bank':
  arch/x86/kernel/cpu/mcheck/mce_amd.c:492: warning: unused variable 'c'

which shows up when kernel is compiled with CONFIG_SMP=n.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
LKML-Reference: <20090915151727.GB21670@alberich.amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>