x86/MTRR: correct inadvertently inverted WC check

The ! clearly got lost by mistake.

Fixes: e9e0eb30d4 ("x86/MTRR: avoid several indirect calls")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/cpu/mtrr/main.c

@@ -316,7 +316,7 @@ int mtrr_add_page(unsigned long base, unsigned long size,
 	}
 
 	/*  If the type is WC, check that this processor supports it  */
-	if ((type == X86_MT_WC) && mtrr_have_wrcomb()) {
+	if ((type == X86_MT_WC) && !mtrr_have_wrcomb()) {
 		printk(KERN_WARNING
 		       "mtrr: your processor doesn't support write-combining\n");
 		return -EOPNOTSUPP;