x86/CPU/AMD: Check vendor in the AMD microcode callback

Commit in Fixes added an AMD-specific microcode callback. However, it
didn't check the CPU vendor the kernel runs on explicitly.

The only reason the Zenbleed check in it didn't run on other x86 vendors
hardware was pure coincidental luck:

  if (!cpu_has_amd_erratum(c, amd_zenbleed))
	  return;

gives true on other vendors because they don't have those families and
models.

However, with the removal of the cpu_has_amd_erratum() in

  05f5f739 ("x86/CPU/AMD: Drop now unused CPU erratum checking function")

that coincidental condition is gone, leading to the zenbleed check
getting executed on other vendors too.

Add the explicit vendor check for the whole callback as it should've
been done in the first place.

Fixes: 522b1d69 ("x86/cpu/amd: Add a Zenbleed fix")
Cc: <stable@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231201184226.16749-1-bp@alien8.de

arch/x86/kernel/cpu/amd.c

@@ -1320,6 +1320,9 @@ static void zenbleed_check_cpu(void *unused)
 
 void amd_check_microcode(void)
 {
+	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
+		return;
+
 	on_each_cpu(zenbleed_check_cpu, NULL, 1);
 }