ssb: use DEVICE_ATTR_ADMIN_RW() helper macro

Use DEVICE_ATTR_ADMIN_RW() helper macro instead of DEVICE_ATTR(), making
it simpler and easier to read.

Because the read and write function names of the sysfs attribute have been
normalized, there is a natural association.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210603082218.11718-2-thunder.leizhen@huawei.com