x86/EFI: fix EFI_MEMORY_WP handling

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -1195,7 +1195,7 @@ void __init efi_init_memory(void)
         }
 
         if ( desc->Attribute & EFI_MEMORY_WP )
-            prot &= _PAGE_RW;
+            prot &= ~_PAGE_RW;
         if ( desc->Attribute & EFI_MEMORY_XP )
             prot |= _PAGE_NX;