firewire: core: improve check for local node

My recently added test for a device being local in fw-cdev.c got it
slightly wrong:  Comparisons of node IDs are only valid if the
generation is current, which I forgot to check.  Normally, serialization
by card->lock takes care of this, but a device in FW_DEVICE_GONE state
will necessarily have a wrong generation and invalid node_id.

The "is it local?" check is made 100% correct and simpler now by means
of a struct fw_device flag which is set at fw_device creation.

Besides the fw-cdev site which was to be fixed, there is another site
which can make use of the new flag, and an RFC-2734 driver will benefit
from it too.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>