sd-device: do not use ::subsystem member directly

The value is set dynamically when sd_device_get_subsystem() is called
first time.

Fixes the following issue:
```
$ build/udevadm test /sys/class/block/dm-1
...
Assertion '_subsystem' failed at src/libsystemd/sd-device/sd-device.c:767, function device_set_subsystem(). Aborting.
Program received signal SIGABRT, Aborted.
```