x86/microcode/AMD: Handle the case of no BIOS microcode

commit fcf8239a upstream.

Machines can be shipped without any microcode in the BIOS. Which means,
the microcode patch revision is 0.

Handle that gracefully.

Fixes: 94838d23 ("x86/microcode/AMD: Use the family,model,stepping encoded in the patch ID")
Reported-by: Vítek Vávra <vit.vavra.kh@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>