libertas: use DEVICE_ATTR_RW macro

Use DEVICE_ATTR_RW 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/20210523040339.2724-1-yuehaibing@huawei.com