MIPS: Loongson: Fix return value of loongson_hwmon_init

MIPS: Loongson: Fix return value of loongson_hwmon_init

When call function hwmon_device_register failed, use the actual
return value instead of always -ENOMEM.

Fixes: 64f09aa967e1 ("MIPS: Loongson-3: Add CPU Hwmon platform driver")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.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