PNP: use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO macro helper instead of plain DEVICE_ATTR, which makes
the code a bit shorter and easier to read.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>