x86/microcode/AMD: Check patch level only on the BSP

Check final patch levels for AMD only on the BSP. This way, we decide
early and only once whether to continue loading or to leave the loader
disabled on such systems.

Simplify a lot.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/20170120202955.4091-13-bp@alien8.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>