firewire: core: optimize card shutdown

This fixes a regression by "firewire: keep highlevel drivers attached
during brief connection loss":  There were 2 seconds unnecessary waiting
added to the shutdown procedure of each controller.

We use card->link as status flag to signal the device handler that there
is no use to wait for a come-back.

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