sd-device: make sd_device_get_devtype() return 0 on success again

sd-device: make sd_device_get_devtype() return 0 on success again

This partially reverts the commit 730b76bd2cd5f0866baa738ae283e3b62544a28f.

Before the commit, the function returned 0 on success, but the commit
made the function always return 1, as if device->devtype is NULL, the
function returns -ENOENT in the above.

Fortunately, udev_device_get_devtype() does not propagate any
non-negative value from sd_device_get_devtype(). Hence, hopefully we can
safely revert the change.