wlcore: use DEVICE_ATTR_<RW|RO> macro

Use DEVICE_ATTR_<RW|RO> helper instead of plain DEVICE_ATTR,
which makes the code a bit shorter and easier to read.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210523033538.25568-1-yuehaibing@huawei.com