x86/spec-ctrl: Honour spec-ctrl=0 for unpriv-mmio sub-option

This was an oversight from when unpriv-mmio was introduced.

Fixes: 8c24b70fed ("x86/spec-ctrl: Add spec-ctrl=unpriv-mmio")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/spec_ctrl.c

@@ -122,6 +122,7 @@ static int __init cf_check parse_spec_ctrl(const char *s)
             opt_l1d_flush = 0;
             opt_branch_harden = false;
             opt_srb_lock = 0;
+            opt_unpriv_mmio = false;
         }
         else if ( val > 0 )
             rc = -EINVAL;