firewire: ohci: access bus_seconds atomically

In the unlikely event that card->driver->get_bus_time() is called during
a cycle64Seconds interrupt, we could read garbage unless atomic accesses
are used.

The switch to atomic ops requires to change the 64 seconds counter from
unsigned to signed, but this shouldn't matter to the end result.

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