Use DEFINE_SHOW_ATTRIBUTE macro to simplify the code.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Link: https://lore.kernel.org/r/20200917125114.103598-1-miaoqinglang@huawei.com


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>