x86/bugs: Simplify SSB cmdline parsing

Simplify the SSB command line parsing by selecting a mitigation directly, as
is done in most of the simpler vulnerabilities.  Use early_param() instead of
cmdline_find_option() for consistency with the other mitigation selections.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/r/20250819192200.2003074-4-david.kaplan@amd.com