x86/bugs: Fix __ssb_select_mitigation() return type

__ssb_select_mitigation() returns one of the members of enum ssb_mitigation,
not ssb_mitigation_cmd; fix the prototype to reflect that.

Fixes: 24f7fc83 ("x86/bugs: Provide boot parameters for the spec_store_bypass_disable mitigation")
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>