x86/bugs: Fix __ssb_select_mitigation() return type

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.

OraBug: 28041771
CVE: CVE-2018-3639

Fixes: 24f7fc83b9204 ("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>
(cherry picked from commit d66d8ff3d21667b41eddbe86b35ab411e40d8c5f)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
 Conflicts:
	arch/x86/kernel/cpu/bugs.c
[It is called bugs_64.c]

Signed-off-by: Brian Maly <brian.maly@oracle.com>