firewire: fw-ohci: don't leak dma memory on module removal

The transmit and receive context dma memory was not being freed on
module removal.  Neither was the config rom memory.  Fix that.

The ab->next assignment is pure paranoia.

Signed-off-by: Jay Fenlason <fenlason@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>