firewire: ohci: properly clear posted write errors

To remove the error information from the controller's queue and to allow
more posted writes, the driver has to read the failed posted write
address before clearing the postedWriteErr interrupt bit.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
(Stefan R:) The spec is somewhat fuzzy about the actual requirements.
To err on the safe side, let's do these two read accesses.

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