arm/xen: Adjust one function call together with a variable assignment

The script "checkpatch.pl" pointed information out like the following.

ERROR: do not use assignment in if condition

Thus fix the affected source code place.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

arch/arm/xen/p2m.c

@@ -153,7 +153,8 @@ bool __set_phys_to_machine_multi(unsigned long pfn,
 	p2m_entry->mfn = mfn;
 
 	write_lock_irqsave(&p2m_lock, irqflags);
-	if ((rc = xen_add_phys_to_mach_entry(p2m_entry)) < 0) {
+	rc = xen_add_phys_to_mach_entry(p2m_entry);
+	if (rc < 0) {
 		write_unlock_irqrestore(&p2m_lock, irqflags);
 		return false;
 	}