LoongArch: mm: Remove the unneeded result variable

Return the value pa_to_nid() directly instead of storing it in another
redundant variable.
Reported-by: NZeal Robot <zealci@zte.com.cn>
Signed-off-by: Nye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: NHuacai Chen <chenhuacai@loongson.cn>

arch/loongarch/mm/init.c

@@ -146,10 +146,7 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
 #ifdef CONFIG_NUMA
 int memory_add_physaddr_to_nid(u64 start)
 {
-	int nid;
-
-	nid = pa_to_nid(start);
-	return nid;
+	return pa_to_nid(start);
 }
 EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
 #endif