Use DEFINE_SHOW_ATTRIBUTE macro to simplify the code.

Link: https://lore.kernel.org/r/20200919025202.17531-1-miaoqinglang@huawei.comReviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Acked-by: Arun Easi <aeasi@marvell.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>