x86/xen: fix SLS mitigation in xen_hypercall_iret()

The backport of upstream patch a2796dff ("x86/xen: don't do PV iret
hypercall through hypercall page") missed to adapt the SLS mitigation
config check from CONFIG_MITIGATION_SLS to CONFIG_SLS.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>