x86/microcode: Remove noisy AMD microcode warning

AMD processors will never support /dev/cpu/microcode updating so
just silently fail instead of printing out a warning for every
cpu.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Borislav Petkov <borislav.petkov@amd.com>
Link: http://lkml.kernel.org/r/1328552935-965-1-git-send-email-prarit@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>