firewire: ohci: Remove unused function

Remove the function ar_prev_buffer_index() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>