s390/speculation: Fix build error caused by bad backport

The backport of commit 0336e04a6520 ("s390/speculation: Support
'mitigations=' cmdline option") introduces a build error. Fix it up.

Fixes: 91788fcb ("s390/speculation: Support 'mitigations=' cmdline option")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>