Use DEFINE_SHOW_ATTRIBUTE macro to simplify the code.

Along with this change, we get additionally:
        .owner          = THIS_MODULE,
        .llseek         = seq_lseek,

1. The llseek method is used to change the current read/write position
in a file which can be ignored if you don't use it.
2. The owner is not even a method. Instead, it is a pointer to the
module that “owns” this structure; it is used by the kernel to maintain
the module's usage count which can be ignored.
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Link: https://lore.kernel.org/r/20200917125113.103550-1-miaoqinglang@huawei.comSigned-off-by: Krzysztof Kozlowski <krzk@kernel.org>