x86/bugs: Fix SRSO reporting on Zen1/2 with SMT disabled

1f4bb068 ("x86/bugs: Restructure SRSO mitigation") does this:

  if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) {
          setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
          srso_mitigation = SRSO_MITIGATION_NONE;
          return;
  }

and, in particular, sets srso_mitigation to NONE. This leads to
reporting

  Speculative Return Stack Overflow: Vulnerable

on Zen2 machines.

There's a far bigger confusion with what SRSO_NO means and how it is
used in the code but this will be a matter of future fixes and
restructuring to how the SRSO mitigation gets determined.

Fix the reporting issue for now.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: David Kaplan <david.kaplan@amd.com>
Link: https://lore.kernel.org/20250513110405.15872-1-bp@kernel.org