firewire: ohci: Move code from the bus reset tasklet into a workqueue

Code inside bus_reset_work may now sleep. This is a prerequisite to
support a phy from Texas Instruments cleanly. The patch to support this
phy will be submitted later.

Signed-off-by: Stephan Gatzka <stephan@gatzka.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>