x86/bugs: Avoid AUTO after the select step in the retbleed mitigation

The retbleed select function leaves the mitigation to AUTO in some cases.
Moreover, the update function can also set the mitigation to AUTO. This
is inconsistent with other mitigations and requires explicit handling of
AUTO at the end of update step.

Make sure a mitigation gets selected in the select step, and do not change
it to AUTO in the update step. When no mitigation can be selected leave it
to NONE, which is what AUTO was getting changed to in the end.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250611-eibrs-fix-v4-1-5ff86cac6c61@linux.intel.com