x86, microcode, AMD: Replace vmalloc+memset with vzalloc

We don't have to do memset() ourselves after vmalloc() when we have
vzalloc(), so change that in
arch/x86/kernel/microcode_amd.c::get_next_ucode().

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>