x86/xen: clean up clang build warning

In the case where sizeof(maddr) != sizeof(long) p is initialized and
never read and clang throws a warning on this.  Move declaration of
p to clean up the clang build warning:

warning: Value stored to 'p' during its initialization is never read
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/include/asm/xen/hypercall.h

@@ -552,13 +552,13 @@ static inline void
 MULTI_update_descriptor(struct multicall_entry *mcl, u64 maddr,
 			struct desc_struct desc)
 {
-	u32 *p = (u32 *) &desc;
-
 	mcl->op = __HYPERVISOR_update_descriptor;
 	if (sizeof(maddr) == sizeof(long)) {
 		mcl->args[0] = maddr;
 		mcl->args[1] = *(unsigned long *)&desc;
 	} else {
+		u32 *p = (u32 *)&desc;
+
 		mcl->args[0] = maddr;
 		mcl->args[1] = maddr >> 32;
 		mcl->args[2] = *p++;