x86, microcode: Fix accessing dis_ucode_ldr on 32-bit

We should be accessing it through a pointer, like on the BSP.

Tested-by: Richard Hendershot <rshendershot@mchsi.com>
Fixes: 65cef131 ("x86, microcode: Add a disable chicken bit")
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Borislav Petkov <bp@suse.de>