MIPS: Loongson: Fix potential NULL dereference in loongson3_platform_init()

If kzalloc fails, it should return -ENOMEM, otherwise may trigger a NULL
pointer dereference.

Fixes: 3adeb256

 ("MIPS: Loongson: Improve LEFI firmware interface")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org